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Concept

Disease is a term for any condition that impairs the normal functioning of an organism or body. Although plants and animals also contract diseases, by far the most significant disease-related areas of interest are those conditions that afflict human beings. They can be divided into three categories: intrinsic, or coming from within the body; extrinsic, or emerging from outside it; and of unknown origin. Until the twentieth century brought changes in the living standards and health care of industrialized societies, extrinsic diseases were the greater threat; today, however, diseases of intrinsic origin are much more familiar. Among them are stress-related diseases, autoimmune disorders, cancers, hereditary diseases, glandular conditions, and conditions resulting from malnutrition. There are also illnesses, such as Alzheimer's disease, whose causes remain essentially unknown.

How It Works

Classifying Diseases

Any condition that impairs the normal functioning of an organism can be called a disease. In the human organism, as in all others, there are certain basic requirements, which in the human body include the need for a certain proper amount of oxygen, acidity, salinity (salt content), nutrients, and so on. These conditions must all be maintained within a very narrow range, and any deviation can bring about disease.

Diseases can be classified into three general groups. There are conditions that are infectious, or extrinsic, meaning that they are caused by an infection through which a virus, bacterium, or other parasite enters the body. Infectious diseases, infections, and the immune system that usually protects us against them are discussed elsewhere in this book. Our attention in the present context will be devoted to the other two broad categories—noninfectious, or intrinsic, diseases and diseases of unknown origin.

Classifying Intrinsic Diseases

There are several basic varieties of intrinsic disease, or conditions that are neither contagious nor communicable. These varieties are listed in the next few paragraphs. The essay Noninfectious Diseases includes a discussion of other systems for classifying diseases of either the intrinsic or the extrinsic variety.

Hereditary diseases: diseases that are genetic, meaning that they are passed down from generation to generation. An example, discussed in Noninfectious Diseases, is hemophilia. Heredity is not a "cause," and some of the diseases of unknown origin may be transmitted from parent to offspring. Some forms of cancer are hereditary as well, as are other conditions discussed elsewhere in this book. (See Nonifectious Diseases, Mutation, and Heredity.)

Glandular diseases: Conditions involving a gland—that is, a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Examples include diabetes mellitus, examined in Noninfectious Diseases, as well as various kidney and liver diseases, among them, hepatitis and jaundice. Goiter, a swelling in the neck area caused by a diet poor in iodine, is both a glandular and a dietary condition, a fact that illustrates the overlap between disease types.

Dietary diseases: These are all illnesses that relate to nutrient deficiencies—either an overall lack of adequate nutrition (i.e., malnutrition) or the absence of a key nutrient. Examples include pellagra, scurvy, and rickets, all of which are vitamin deficiencies, as well as kwashiorkor, which brings about a swollen belly and is caused by a lack of protein. Vitamin deficiencies are discussed in Vitamins, and kwashiorkor and other varieties of malnutrition are examined in Nutrients and Nutrition.

Cancers: Cancer is not just one disease but some 100 conditions. Its two main characteristics are uncontrolled growth of diseased cells in the human body and migration of the disease from the original site to distant sites within the body. If the spread is not controlled, cancer can result in death. (See Noninfectious Diseases for more.)

Stress-related diseases: Some heart conditions are hereditary or glandular, but quite a few diseases of the heart and circulatory system are exacerbated by stress. Examples include heart murmurs, hardening of the arteries, and varicose veins. We will examine heart disease and the general effects of stress shortly.

Autoimmune diseases: This is a particularly terrifying category of disease, because it involves a rejection of the body itself by the body's own immune system. Autoimmune diseases, examples of which include lupus and rheumatoid arthritis, are discussed in The Immune System.

Diseases of Unknown Origin

Finally, there are diseases for which there is no known cause. In some cases, it is possible that heredity, diet, or some other aspect of human existence has a role, but it is not certain. And even if, say, heredity plays a part, the exact hereditary factors are not established. In any case, many of the categories of disease we have listed do not amount to "causes," but rather are types of disease. Moreover, some diseases classifiable in one of the listed categories also belong in the ranks of the diseases with unknown causes. For instance, many autoimmune diseases are mysterious to scientists. Likewise, chronic fatigue syndrome, considered a disease of unknown origin, is obviously a stress-related disorder, while fibromyalgia, characterized by sore muscles and tissues, may be stress-related as well. Two brain diseases of unknown origin, Creutzfeldt-Jakob and Alzheimer's disease, are discussed near the conclusion of this essay.

Real-Life Applications

A Changing Threat

At one time the diseases that posed the greatest threat to human survival were infectious ones, such as the Black Death (actually a combination of bubonic and pneumonic plague), which killed about a third of Europe's population during the period from 1347 to 1351. Plagues or epidemics, in fact, are among the persistent themes in history, punctuating the fall of empires and the rise of others.

A plague that struck the eastern Roman (Byzantine) Empire in the sixth century, for instance, brought an end to a plan by the great Justinian I to reconquer the Italian peninsula and restore Roman rule in western Europe. It also spelled the beginning of the end of Byzantine glory (though the empire hung on until 1453) and opened the way for the rise of Islam and Muslim influence over the Mediterranean. Thus, the course of history up to the present day, including the events of the European Middle Ages, the Crusades, and even the modern-day conflict between the West and Islamic terrorists, can be traced in part back to a plague in about A.D. 540.

Wherever people have gathered in large numbers, infectious diseases have arisen. Smallpox and chicken pox, cholera and malaria, diphtheria and scarlet fever, influenza and polio—these and many other diseases have threatened the very survival of whole populations, bringing about a collective death toll that dwarfs that of twentieth-century wars and genocide. Yet it was in the twentieth century—ironically, the era when humans discovered the capacity to kill themselves in truly frightening numbers through world wars, nuclear weaponry, and totalitarian social experiments—that the threat of infectious diseases began to recede.

Thanks to successful vaccination programs, many infectious diseases are largely a thing of the past. This is true even of smallpox, a scourge that effectively ended in 1978 thanks to a United Nations inoculation program, but which reemerged as a potential threat of biological terrorism in the hands of political terrorists, such as the Islamic terrorist Osama bin Laden. It would be difficult for bin Laden's al-Qaeda organization to acquire samples of the virus, however; they are stored in only four or five laboratories worldwide (kept there for the purpose of making more vaccine if needed) and remain under heavy guard.

The Rise of Intrinsic Diseases

Rather than infectious diseases, the much greater threat today is in the form of intrinsic diseases, or ones that are neither communicable nor contagious. The leading causes of death in the United States are as follows:

• Heart disease
• Cancer
• Stroke
• Chronic obstructive lung diseases (e.g., emphysema)
• Accidents (motor vehicle or other)
• Pneumonia and influenza
• Diabetes mellitus
• Suicide
• Kidney disease
• Chronic liver disease and cirrhosis

Note that the one item on the list that is not an intrinsic disease and is not disease-related at all: accidents. After that is the first extrinsic disease entry on the list, number 6, pneumonia and the closely related condition influenza. Number 8, of course, is not related to disease—at least not physical disease. The high incidence of suicide, with 11.1 such deaths per 100,000 population, probably reflects the fact that the United States is an industrialized, wealthy nation. Ironically, people who are eking out a living, struggling for survival, are far less likely to end it all voluntarily.

Also reflective of America's high level of development is the overwhelming preponderance of intrinsic, noninfectious diseases on the list. Unquestionably, the greatest threat to human health today takes the form of noninfectious diseases, such as heart disease, cancer, and diseases of the circulatory system. This is true only in the industrialized world, however: whereas only about 25% of all patients who visit doctors in the United States do so because of infectious diseases, more than two-thirds of all deaths worldwide are caused by infectious diseases, such as malaria.

Stress and Heart Disease

Stress, simply put, is a condition of mental or physical tension brought about by internal or external pressures. Many events can cause stress: something as simple as taking a test or driving through rush-hour traffic or as traumatic as the death of a loved one or contracting a serious illness. Stress may be short-lived, as when facing a particular deadline, or it may be the ongoing, crippling stress related to a job that is slowly killing the victim.

People who experience severe traumas, such as soldiers in combat, may experience a condition called post-traumatic stress disorder (PTSD). This condition first came to public attention after World War I, a war that completely dwarfed all preceding conflicts in its intensity and brutality. Formerly bright-eyed, optimistic youths came home behaving like madmen or nervous wrecks, and soon the condition gained the nickname shell shock. (Actually, shell shock dated back more than 50 years, to what might be regarded as the first modern war in the West—the first "total war" involving relatively sophisticated weaponry and a fully engaged citizenry: America's Civil War, from which combatants returned home with a condition known as "soldier's heart.")

Effects of Stress

Whereas PTSD has a distinct psychological dimension, in many stress-related diseases there is not as obvious a link between mental states and bodily disorders. Nonetheless, it is clear that stress kills. Some of the physical signs of stress are a dry mouth and throat, headaches, indigestion, tremors, muscle tics, insomnia, and a tightness of the muscles in the shoulders, neck, and back. Emotional signs of stress include tension, anxiety, and depression. During stress, heart rate quickens, blood pressure increases, and the body releases the hormone adrenaline, which speeds up the body's metabolism. Stress may disrupt homeostasis, an internal bodily system of checks and balances, leading to a weakening of immunity.

Diseases and conditions associated with stress include adult-onset diabetes (see Noninfectious Diseases), ulcers, high blood pressure, asthma, migraine headaches, cancer, and even the common cold. The last, of course, is an infectious illness, but because stress impairs the immune system, it can leave a person highly susceptible to infection. Furthermore, medical researchers have determined that long-term stress causes the accumulation of fat, starch, calcium, and other substances in the linings of the blood vessels. This condition ultimately results in heart disease.

Heart Diseases

The human heart weighs just 10.5 oz. (300 g), but it contracts more than 100,000 times a day to drive blood through about 60,000 mi. (96,000 km) of vessels. An average heart will pump about 1,800 gal. (6,800 l) of blood each day. With exercise, that amount may increase as much as six times. In an average lifetime the heart will pump about 100 million gal. (380 million l) of blood. The heart is divided into four chambers: the two upper atria and the two lower ventricles. The wall that divides the right and left sides of the heart is the septum. Movement of blood between chambers and in and out of the heart is controlled by valves that allow transit in only one direction.

Given its importance to human life, it follows that heart disease is an extremely serious condition. Among the many illnesses that fall under the general heading of heart disease is congenital heart disease, a term for any defect in the heart that is present at birth. About one of every 100 infants is born with some sort of heart abnormality, the most common form being the atrial septal defect, in which an opening in the septum allows blood from the right and left atria to mix.

Coronary heart disease, also known as coronary artery disease, is the most common form of heart disease. A condition termed arteriosclerosis, in which there is a thickening of the artery walls, or a variety of arteriosclerosis known as atherosclerosis results when fatty material, such as cholesterol, accumulates on an artery wall. This forms plaque, which obstructs blood flow. When the obstruction occurs in one of the main arteries leading to the heart, the heart does not receive enough blood and oxygen, and its muscle cells begin to die.

Creutzfeldt-Jakob Disease

A particularly frightening category of unexplained diseases includes those that attack and destroy the brain. Among them are two conditions named after German scientists: the psychiatrists Alfons Maria Jakob (1884-1931) and Hans Gerhard Creutzfeldt (1885-1964) and the neurologist Alois Alzheimer (1864-1915). Creutzfeldt-Jakob disease, fortunately, is a rare condition. The disease, first described by the two doctors in the 1920s, initially shows itself with the loss of memory, and within a few weeks it progresses to visual problems, loss of coordination, and seizure-like muscular jerking. Death usually follows within a year.

It appears that Creutzfeldt-Jakob disease ensues when a certain protein in the brain, known as prion protein, changes into an abnormal form. As to what causes that change, scientists remain in the dark. The disease attacks about one of a million people worldwide, and victims are typically about 50-75 years of age. During the 1990s something strange happened: the disease began affecting relatively large numbers of young people in the United Kingdom. A 1996 report of British medical experts, however, linked the surge in Creutzfeldt-Jakob cases to what might be considered a dietary condition: bovine spongiform encephalopathy, or mad cow disease, contracted from eating cattle with a form of prion disease. The only way to contract such a condition, however, is by eating the brain or spinal cord of an affected cow, something that could only happen in the case of hamburger or sausage, in which one does not always know what one is getting. The cows themselves got the disease from eating feed tainted with by-products of other cows, and as a result of the outbreak, Great Britain issued wide-ranging controls prohibiting the production of feed containing any materials from cows. (These particular feed-production practices were never common in the United States.)

Alzheimer's Disease

Whereas Creutzfeldt-Jakob disease is a little-known condition, Alzheimer's disease is all too familiar to the families of the more than four million sufferers in America today. Note the reference to the families rather than the victims themselves: one of the most devastating aspects of Alzheimer's disease is the patient's progressive loss of contact with reality, such that a patient in an advanced stage does not even know that he or she has the disease. A progressive brain disease that brings about mental deterioration, Alzheimer's disease is signaled by symptoms that include increasingly poor memory, personality changes, and a loss of concentration and judgment. Although most victims are older 65 years, Alzheimer's is not a normal result of aging. Up until the 1970s people assumed that physical and mental decline were normal and unavoidable features of old age and dismissed such cases of deterioration as "senility." Yet as early as 1906, Alzheimer himself discovered evidence that pointed in a different direction.

In that year Alzheimer was studying a 51-year-old woman whose personality and mental abilities were obviously deteriorating. She forgot things, became paranoid, acted strangely, and just over four years after he began working with her, she died. Following an autopsy, Alzheimer examined sections of her brain under a microscope and noted deposits of an unusual substance in her cerebral cortex—the outer, wrinkled layer of the brain, where many of the higher brain functions, such as memory, speech, and thought, originate. The substance Alzheimer saw under the microscope is now known to be a protein called beta-amyloid. About 75 years later scientists and physicians began to recognize a strong link between "senility" and the condition Alzheimer had identified. Since then, the public has become more aware of the disease, especially since Alzheimer's disease has stricken such well-known figures as the former president Ronald Reagan (1911-) and the actress Rita Hayworth (1918-1987).

The Impact of Alzheimer's Disease

A slight decline in short-term memory (as opposed to long-term memories of childhood and the like) is typical even in healthy elderly adults, but the memory loss seen in Alzheimer's disease is much more severe. As years pass, memory loss becomes greater, and personality and behavioral changes occur. Later symptoms include disorientation, confusion, speech impairment, restlessness, irritability, and the inability to care for oneself. Although victims may remain physically healthy for years, the progressive decline of their mental faculties is ultimately fatal: eventually, the brain loses the ability to control basic physical functions, such as swallowing. Persons with Alzheimer's disease typically live between five and ten years after diagnosis, although improvements in health care in recent years have enabled some victims to survive for 15 years or even longer.

Improvements in health care also may help explain the fact that the numbers of Alzheimer victims are growing. Medical discoveries of the twentieth century served to prolong life greatly, such that there are far more people alive today who are 65 years of age or older than there were in 1900. More accurate reporting no doubt plays a part as well. Whereas about 2.5 million cases were reported throughout the 1970s, by the end of the twentieth century there were some four million living Alzheimer victims, and by the mid-twenty-first century that number is expected to climb to the range of 13 million if physicians do not find a cure. Meanwhile, Alzheimer's causes the deaths of more than 100,000 American adults each year and costs $80-90 billion annually in health-care expenses.

Understanding Alzheimer

It is not a simple procedure to diagnose Alzheimer's disease, and despite all the medical progress since the time of Alois Alzheimer, the "best" method for determining whether someone has the condition is hardly a good one. The only possible physical procedure for definitively diagnosing Alzheimer's disease is to open the skull and remove a sample of brain tissue for microscopic examination. This is rarely done, of course, because brain surgery is far too drastic a procedure for simply obtaining a sample of tissue.

The immediate cause of Alzheimer's is the death of brain cells and a decrease in the connections between those cells that survive. But what causes that? Many scientists today believe that the presence of beta-amyloid protein is a cause in itself, while others maintain that the appearance of the protein is simply a response to some other, still unknown phenomenon. Researchers have found that a small percentage of Alzheimer cases apparently are induced by genetic mutations, but most cases result from unknown factors. Various risk factors have been identified, but they are not the same as causes; rather, a risk factor simply means that if a person has x, he or she is more likely to have y. Risk factors for Alzheimer's include exposure to toxins, head trauma (former president Reagan suffered a serious head injury before the onset of Alzheimer's disease), Down syndrome (a genetic disorder that causes mental retardation), age, and even gender (women are more likely than men to suffer from Alzheimer's disease).

Familial Alzheimer's disease, an inherited form, accounts for about 10% of cases. Approximately 100 families in the world are known to have rare genetic mutations that are linked with early onset of symptoms, and some of these families have an aggressive form of the disease in which symptoms appear before age 40. The remaining 90% of cases may be caused by various combinations of genetic and as yet undefined environmental factors.

Where to Learn More

Centers for Disease Control and Prevention (Web site). <http://www.cdc.gov/>.

DeSalle, Rob. Epidemic!: The World of Infectious Disease. New York: New Press, 1999.

Diseases, Disorders and Related Topics. Karolinska Institutet/Sweden (Web site). <http://www.mic.ki.se/Diseases/>.

Environmental Diseases from A to Z. National Institute of Environmental Health Sciences/National Institutes of Health (Web site). <http://www.niehs.nih.gov/external/a2z/home.htm>.

Epidemiology. University of Minnesota, Crookston (Web site). <http://sunny.crk.umn.edu/courses/biolknut/1020/micro3>.

Ewald, Paul W. Plague Time: How Stealth Infections Cause Cancers, Heart Disease, and Other Deadly Ailments. New York: Free Press, 2000.

Garrett, Laurie. The Coming Plague: Newly Emerging Diseases in a World out of Balance. New York: Farrar, Straus, Giroux, 1994.

Moore, Pete. Killer Germs: Rogue Diseases of the Twenty-First Century. London: Carlton Books, 2001.

National Cancer Institute, National Institutes of Health (Web site). <http://www.nci.nih.gov/>.

Oncolink: University of Pennsylvania Cancer Center (Web site). <http://oncolink.upenn.edu/>.

Oldstone, Michael B. A. Viruses, Plagues, and History. New York: Oxford University Press, 1998.

"Plant and Animal Bacteria Diseases." University of Texas Institute for Cellular and Molecular Biology (Web site). <http://biotech.icmb.utexas.edu/pages/science/bacteria.html#disease>.

World Health Organization (Web site). <http://www.who.int/home-page/>.

A deleterious set of responses which occurs at the subcellular level, stimulated by some injury, and which is often manifested in altered structure or functioning of the affected organism. With advances in understanding and the development of sensitive probes, it has become clear that the fundamental causes of diseases are based on biochemical and biophysical responses within the cell. These responses are now being categorized and, slowly, the mechanisms are being understood.

The term homeostasis refers to functionàl equilibrium in an organism and to the processes that maintain it. There is a range of responses that is considered normal. If cells are pushed to respond beyond these limits, there may be an increase, a decrease, or a loss of normal structure or function. These changes may be reversible or irreversible. If irreversible, the cells may die. Thus, subcellular changes may be reflected in altered tissues, organs, and consequently organisms, and result in a condition described as diseased. See also Homeostasis.

Lesions are the chemical and structural manifestations of disease. Subjective manifestations of a disease process such as weakness, pain, and fatigue are called symptoms. The objective measurable manifestations such as temperature, blood pressure, and respiratory rate changes are called signs or physical findings. Changes in the chemical or cellular makeup of an organ, tissue, or fluid of the body or its excretory products are called laboratory findings. To make a diagnosis is to determine the nature of the pathologic process by synthesizing information from these sources evaluated in the light of the patient's history and compared with known patterns of signs and symptoms. In common usage, the term disease indicates a constellation of specific signs and symptoms attributable to altered reactions in the individual which are produced by agents that affect the body or its parts.

Etiology is the study of the cause or causes of a disease process. Although a disease may have one principal etiologic agent, it is becoming increasingly apparent that there are several factors involved in the initiation of a disease process. Susceptibility of the individual is an ever present variable. The etiologic factors can conveniently be divided into two categories (see table). One group consists of endogenous (internal; within the body) factors, and may originate from errors in the genetic material. The other category of etiologic factors is exogenous (environmental). These account for the majority of disease reactions. Exogenous factors include physical, chemical, and biotic agents.

Pathogenesis refers to the mechanisms by which the cell, and consequently the body, responds to an etiologic agent. It involves biochemical and physiological responses which are reflected in ultrastructural, microscopic, or gross anatomic lesions. There are a limited number of ways in which cells respond to injury. The nature of the response is modified by the nature of the agent, dose, portal of entry, and duration of exposure, as well as many host factors such as age, sex, nutritional state, and species and individual susceptibility.

The diseases which are important in causing human death have changed in the last 80 years. In 1900 six of the ten leading causes of death in the United States were infectious (biotic) agents. At present, only one of the ten leading causes of death in the United States, influenza and pneumonia, is due to biotic agents. While most of the biotic causes of diseases were being brought under control, continued population growth (in large part, a consequence of the control of infectious disease) and the remarkable growth of industrialization have been associated with an increased prevalence of diseases caused by physical and chemical agents. These include cancer, cirrhosis, and cardiovascular disease.

General principles of the organism's response to toxic substances, some of which occur naturally in the environment, have evolved from a great number of investigations of agent-host interaction. They are: (1) All substances entering the organism are toxic; none is harmless. Dose rate of exposure and route of entry into the body determine whether a toxic response will occur or not. (2) All agents evoke multiple responses. (3) Most of the biological responses are undesirable, leading to the development of pathological changes. (4) A given dose of an agent does not produce the same degree of response in all individuals. Thus, when disease is viewed as interaction between the environment and the individual, the control of disease is largely the management of the environmental causes of disease.

Disease can be broadly defined as any illness or sickness that impairs or disrupts the normal functioning of the human body. Definitions of disease, however, have varied over time and place, and diagnostic categories vary. We will be concerned here with concepts of disease in the Western medical tradition. Medicine itself can be defined as ‘the science and art concerned with the cure, alleviation, and prevention of disease, and with the restoration and preservation of health’. Thus the definition, treatment, and prevention, of disease is intimately associated with the development of medicine itself, and several specialist areas of medicine are specifically concerned with broad aspects of disease. For example nosology, the classification of diseases, was for many centuries the cornerstone of medicine; therapeutics is the treatment of disease; diagnosis is the art the physician uses to determine from the patients' signs and symptoms what the underlying causative mechanism may be; epidemiology is the study of disease in populations.

The Hippocratic tradition maintained that diseases were physiological, arising from an imbalance between the four humours, the correct balance of which maintained health. Each individual had a unique humoral balance which could be easily disrupted by conditions such as cold, biting winds, poor air, or injudicious eating. It was Paracelsus (c.1492-1541) who provided an alternative to such classical ideas by suggesting that disease was the product of active agents independent of the human patient. The English Hippocrates, Thomas Sydenham (1624-89), also believed that diseases were specific entities, which might be manifest in variable ways in individual patients, but which could be recognized by observant clinicians. Sydenham also advocated that specific remedies could be applied to each such disease, his favourite example being the prescription of Peruvian bark for intermittent fever or ague. It was during the eighteenth century that the classification of diseases became a dominant part of medicine, such taxonomies often being based on the presenting symptoms. Increasing attention to pathology and cellular mechanisms during the nineteenth century provided additional criteria whereby diseases could be described, recognized, and treated.

An increasing array of diagnostic techniques have become available during the twentieth century, and these have made it possible for even more diseases, syndromes, and conditions to be classified, and the International Classification of Disease goes some way to providing international standardization in the categorization of modern disease.

Disease categories

One broad classification divides diseases into two principle categories, defining disease as primarily congenital (present at birth) or acquired subsequently. The acquired category can be further subdivided to include infectious, neoplastic, traumatic, and degenerative diseases, which are not necessarily mutually exclusive. Modern concepts and definitions have obscured many of these classificatory boundaries, and occupational, nutritional and deficiency, autoimmune and allergic, and psychiatric diseases can now be included in contemporary nosologies. As the molecular mechanisms of diseases and their causations are increasingly understood, so classifications are increasingly becoming blurred and overlapping, and many diseases are now recognized as being multi-causal. The following sections will give brief overviews of some broad classes of disease.

Genetics and diseases

Hereditary diseases may be passed down from generation to generation, but are not necessarily genetic disorders unless determined by one or more genes. The experiments on peas by Gregor Mendel (1822-84), on the transfer of characteristics from generation to generation, established the basic principles of heredity. Mendel and his successors determined a number of factors to define genetic inheritance, including its occurrence in known proportions amongst relatives, but not in unrelated individuals, such as in-laws. Applied to human conditions, some diseases were now identified as hereditary, including haemophilia and sickle-cell anaemia — although it had been recognized for centuries that some diseases ‘ran in the family’.

congenital abnormalities are present at birth, though some do not immediately become apparent. They are not necessarily inherited, many in fact being environmentally determined during intrauterine life. It is now known that many common diseases, including heart disease, insulin dependent diabetes, some forms of psychiatric illnesses, and autoimmune diseases, have a genetic component. But these susceptibilities are often triggered by some environmental influence, and represent a complex interaction between nature (genetic makeup) and nurture (external influences). Some forms of cancer are increasingly recognized as having a major genetic component. The genetics of some diseases, such as cystic fibrosis, beta-thalassaemia, and Duchenne's muscular dystrophy, are now well known, and it is possible to screen parents, and unborn fetuses, to detect genetic abnormalities, and to offer termination of an affected fetus. The development of in utero gene therapy offers the hope of treatment, whilst modern medicine can do a great deal to maintain individuals who, in earlier periods, would have died because of their genetic constitution.

Infectious diseases

infectious diseases caused by microorganisms or parasites have been powerful forces in shaping human history. As early humans formed hunter-gatherer societies in about 3000 bc and began to contain and domesticate wild animals, so they became susceptible to the infections carried by the animals with which they now shared their living space. Smallpox, distemper, and measles are amongst the diseases known to have entered human populations at this time. The devastating effects of these diseases gradually became ameliorated as immunity built up in communities — major epidemics (and pandemics) were often caused by the movement of communities where such diseases were endemic into non-immune populations. Urbanization provided fresh opportunities for infectious diseases to flourish and to decimate populations — close living conditions encouraged the transfer of infections; migration into the cities from rural communities, and the dependency of urban populations on the countryside for food, provided fresh avenues for infection. Until almost the end of the nineteenth century, sustaining cities was a constant problem, and the large metropolitan areas of Western civilization were known to be centres of disease, malnutrition and starvation, and ultimately death. Diseases such as smallpox, syphilis, typhoid fever, and whooping cough were all endemic and accounted for a high infant mortality — estimates have suggested for example that the mortality rate for children under 5 was as high as 50% for much of the nineteenth century in the English city of Manchester. By the latter half of that century, the experiments of Robert Koch (1843-1910) in Germany and Louis Pasteur (1822-95) in France, amongst others, increasingly provided evidence that microganisms were the cause of several infectious diseases. Efforts were made to utilize this knowledge in the manufacture of vaccines, preparations of modified or killed bacteria that could be administered to produce a mild form of the disease and to confer immunity. Although the precise mechanisms of how such immunity was created were unknown, a number of therapeutic substances were developed, the biggest breakthrough coming at the end of the nineteenth century with the appearance of serum anti-toxins. Since then, increasing understanding of the underlying cellular mechanisms of immunity and of the biology of microorganisms; the growth and development of the pharmaceutical industry, especially the discovery of antibiotics; and the development of public health measures to prevent and treat infectious diseases, have led to a notable decline in mortality and morbidity from such diseases in the Western world. The same cannot be said for the developing world, and concerns grew at the end of the twentieth century about resistance to antibiotics, the appearance of new infectious killers such as HIV (which causes AIDS), and the re-emergence of drug-resistant forms of diseases such as TB.

Autoimmune diseases

autoimmune diseases occur when elements of the immune system, normally responsible for recognizing and attacking ‘non-self’ cells — such as the microorganisms that cause infectious diseases — fail to distinguish between ‘self’ and ‘non-self’. Such cellular attacks on healthy constituent parts of the body can contribute to a variety of disorders, including myasthenia gravis, some thyroid disorders, and rheumatoid arthritis. There is growing evidence that conditions such as diabetes and multiple sclerosis also have an autoimmune component.

Diseases associated with food: deficiencies, excesses, and intolerances

Deficiency (nutritional) diseases arise from lack of one or more essential nutritional component, such as a vitamin or mineral, in the diet, or because of the body's inability to digest, absorb, or utilize particular nutrients. A nutritional deficiency can also occur if the body's metabolism is abnormal or if essential elements are excessively excreted. Historically, deficiency diseases have arisen in populations forced, by war or famine, to abandon their traditional diets, or by the adoption, perhaps for religious reasons, of a restrictive diet. Expeditions into new territories have always been vulnerable to dietary diseases because of the difficulty of carrying adequate supplies. The most notable example was that of scurvy, and the development of its treatment by eating citrus fruits, which occurred long before the rationale was understood — namely that this corrected a vitamin C deficiency due to lack of fresh fruit and vegetables. Subtle changes to farming or cooking methods can also lead to unexpected deficiencies. One of the best known examples is the use of white (huskless) rice instead of brown (husked) rice. This can lead to beri-beri, particularly prevalent in the Far East, which is characterized by ascending weakness in the legs and accompanying muscle tenderness, and can lead to widespread nerve irritability and congestive heart failure. This is due to a lack of thiamine, a vitamin that is essential for the mechanisms by which energy is released from foodstuffs.

Excess consumption of certain kinds of foods has been shown to be associated with the onset of conditions such as heart disease or diabetes — often exacerbating a genetic predisposition, and thus once more blurring the distinctions between different disease categories. Over consumption of alcohol, cigarette smoking, and taking other damaging drugs can all lead to disease conditions that can be classified as ‘self-inflicted’ (a category that can also include sexually transmitted diseases contracted during unprotected sex).

Food intolerances have been increasingly recognized in the latter part of the twentieth century, as some individuals show anaphylactic responses to particular allergens, such as nuts or dairy products.

Occupational diseases and the effects of pollution

Concern about the workplace as a source of disease has grown, particularly since World War II, as have the specialities of public and occupational health. Safety procedures, including the use of protective clothing, have been proposed to limit workers' exposure to dangerous chemicals or to hazardous practices. Working conditions, such as those for office workers using video display equipment, have received attention, and exposure to noise, bad ventilation, and poorly designed furniture have increasingly been recognized as playing a role in stress. Stress in turn is recognized as contributing to high blood pressure, heart disease, and stroke.

These concerns are not, however, entirely new. Lead poisoning amongst miners was recognized by Hippocrates (c.450 bce), but it was really the impact of the Industrial Revolution that focused the attention of reformers and some physicians on the impact of working conditions on health. A Leeds physician, Charles Thackrah, wrote The effects of arts, trades and professions on health and longevity (1832), which described the occupational hazards attached to numerous trades, including flock dressers, maltsters, coffee grinders, and corn-sillers. The effects of adverse and dangerous conditions on the ordinary working man became an issue for the growing number of trade or labour unions, and reform movements throughout the twentieth century campaigned for safer working conditions and adequate health care and compensation for those injured in the workplace.

Increasingly, however, environmental dangers have been recognized as having wider impact than just at the workplace. There can be pollution from accidental contamination and from large-scale industrial accidents. Disease and disasters can arise from cynical exploitation by manufacturers who ignore concerns for the welfare not only of their own workforce, but also of those living in the vicinity of their production facilities, such as the workers in the asbestos industry or the victims of the Bhopal explosion in India that killed 2000 people. Dispersal of pollutants, by air as after the Chernobyl disaster in Ukraine, or by river systems, can cause disease at vast distances from the original site of contamination.

Psychiatric diseases

Mental illness can refer to disorders in perception, understanding, emotion, and behaviour, and can range from the milder psychological disorders and psychosomatic illness to the severe psychosis. Psychiatric disorders have not always been seen to be the province of the medical profession: theories about demonic possession, for example, have led to religious remedies or persecution. For many centuries doctors had little to do with those classed as ‘insane’. The insane were incarcerated and contained, rather than treated. In the twentieth century increasing acknowledgement of the interplay of social, psychological, and physical factors in the causation of many psychiatric disorders, and the development of specific pharmacological therapies, led to improved care. Here again, the categories of disease classifications have become blurred, as faulty chemical processes in the brain and genetic defects have been shown to account for some manifestations of mental disease.

Degenerative diseases

Ironically, as infectious diseases were increasingly conquered during the twentieth century, degenerative diseases emerged in the West-ern world, primarily affecting the elderly. Degenerative processes can strike in particular organs or tissues, resulting in damaged joints, such as hips and knees; in weakened bones, as a result of osteoporosis; or as degeneration of the brain, causing severe mental deficits, such as dementia. It has been argued, most notably by the epidemiologist, Thomas McKeown (1912-88), that the main risks to life and good health have occurred in 3 distinct historical phases: accidents and injuries; infections; and finally degenerative diseases of longevity, which can include diseases such as Alzheimer's disease, Parkinson's diseases and some cancers.

One of the best known degenerative diseases is Alzheimer's, first described in 1906 by Alois Alzheimer, but then recognized as only a very rare brain disorder associated with cognitive dysfunction. This type of dementia is now the most common acquired progressive brain syndrome, although its cause remains unknown. Recent figures from the US have shown that Alzheimer's affects more than 4% of the over-60s population, whilst prevalence grows to 20% of the over-80s age group. The impact of chronic degenerative disease is felt not only by individuals and families, but also by social welfare and health care systems.

Cancer

Cancer is caused by a breakdown in the normal processes of cell division and multiplication, resulting in uncontrolled cell growth producing a tumour. In the industrialized world, at the beginning of the twenty-first century, it is estimated that one-third of the population will develop cancer, with the probability currently increasing. This is partly because it is predominantly a disease of middle and old age, and as life expectancy has increased, so too has the incidence of cancer. Several cancers are known to have a genetic basis, and also the environmental impact of some pollutants, known as carcinogens, is becoming increasingly well understood.

Iatrogenic diseases

These diseases arise from medical treatment for another condition. Sometimes the problem may be due to recognized undesirable side-effects of therapeutic drugs, or to an unusual, idiosyncratic reaction to a medicament. A scheme of reporting adverse drug reactions, the so-called ‘yellow card scheme’ was introduced in Britain in the early 1970s, in an attempt to identify such reactions. Surgical procedures, when mishaps or infections result, can also inadvertently cause further disease.

— E. M. Tansey

Bibliography

• Kiple, K. F. (ed.) (1993). The Cambridge world history of human disease. Cambridge University Press.
• McKeown, T. (1979). The role of medicine: dream, mirage or nemesis. Oxford University Press.
• McKeown, T. (1988). The origins of human disease. Oxford University Press

See also allergy; drug abuse; environmental toxicology; genetics, human; Islamic medicine; medicine; mind-body interaction; work and the body.

noun

A pathological condition of mind or body: ailment, complaint, disorder, ill, illness, infirmity, malady, sickness. See health/sickness.

Definition: ailment, affliction
Antonyms: health

Any disorder with a characteristic set of signs and symptoms, except that resulting from physical trauma.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

Quotes:

"Is not disease the rule of existence? There is not a lily pad floating on the river but has been riddled by insects. Almost every shrub and tree has its gall, oftentimes esteemed its chief ornament and hardly to be distinguished from the fruit. If misery loves company, misery has company enough. Now, at midsummer, find me a perfect leaf or fruit." - Henry David Thoreau

"We are so fond on one another because our ailments are the same." - Jonathan Swift

"With the modern diseases (once TB, now cancer) the romantic idea that the disease expresses the character is invariably extended to assert that the character causes the disease -- because it has not expressed itself. Passion moves inward, striking and blighting the deepest cellular recesses." - Susan Sontag

"Any important disease whose causality is murky, and for which treatment is ineffectual, tends to be awash in significance." - Susan Sontag

"Disease is not of the body but of the place." - Seneca

"The diseases which destroy a man are no less natural than the instincts which preserve him." - George Santayana

See more famous quotes about Disease

Traditionally defined as a finite abnormality of structure or function with an identifiable pathological or clinicopathological basis, and with a recognizable syndrome or constellation of clinical signs.
This definition has long since been widened to embrace subclinical diseases in which there is no tangible clinical syndrome but which are identifiable by chemical, hematological, biophysical, microbiological or immunological means. The definition is used even more widely to include failure to produce at expected levels in the presence of normal levels of nutritional supply and environmental quality. It is to be expected that the detection of residues of disqualifying chemicals in foods of animal origin will also come to be included within the scope of disease.
For specific diseases see under the specific name, e.g. Aujeszsky's disease, Bang's disease, foot-and-mouth disease.

• air-borne d. — the causative agent is transmitted via the air without the need for intervention by other medium. See also wind-borne disease.
• d. carrier — see carrier, vector.
• clinical d. — see clinical (3).
• d. cluster — a group of animals with the same disease occurs at an unusual level of prevalence for the population as a whole. The cluster may be in space, with high concentrations in particular localities, or in time, with high concentrations in particular seasons or in particular years.
• communicable d. — infectious disease in which the causative agents may pass or be carried from one animal to another directly or indirectly on inanimate objects or via vectors.
• complicating d. — one that occurs in the course of some other disease as a complication.
• constitutional d. — one involving a system of organs or one with widespread signs.
• contagious d. — see communicable disease (above).
• d. control — reducing the prevalence of a disease in a population, including eradication, by chemical, pharmaceutical, quarantine, management including culling, or other means or combinations of means.
• d. control programs — organized routines specifying agents, administration, time and personnel allocations, community support, funding, participation of corporate or government agencies, animal and animal product disposal.
• deficiency d. — a condition due to dietary or metabolic deficiency, including all diseases caused by an insufficient supply of essential nutrients.
• degenerative joint d. — see degenerative joint disease, osteoarthritis.
• demyelinating d. — any condition characterized by destruction of myelin.
• d. determinant — any variable associated with a disease which, if removed or altered, results in a change in the incidence of the disease.
• egg-borne d. — an infectious disease of birds in which the agent is spread via the egg.
• endemic d. — see endemic.
• environmental d. control — control by changing the environment, e.g. draining a swamp, ventilating a barn.
• epidemic d. — see epidemic.
• etiological d. classification — diseases arranged in the order of their etiological agents, e.g. bacterial, mycoplasma.
• exotic d. — a disease that does not occur in the subject country. Said of infectious diseases that may be introduced, e.g. rabies is exotic to the UK, contagious bovine pleuropneumonia is exotic to the USA.
• focal d. — a localized disease.
• fulminant d. — an explosive outbreak in a group or a rapidly developing, peracute development of a disease in an individual. Called also fulminating.
• functional d. — any disease involving body functions but not associated with detectable organic lesion or change.
• generalized d. — one involving all or many body systems; often said of infectious diseases in which there is spread via the bloodstream. See also systemic disease (below).
• glycogen d. — any of a group of genetically determined disorders of glycogen metabolism, marked by abnormal storage of glycogen in the body tissues. See also glycogen storage disease.
• heavy chain d. — see heavy-chain disease.
• hemolytic d. of newborn — see alloimmune hemolytic anemia of the newborn.
• hemorrhagic d. of newborn — see neonatal hemorrhagic disease.
• d. history — that part of a patient's history which relates only to the disease from which the patient is suffering.
• holoendemic d. — most animals in the population are affected.
• hyperendemic d. — the rate of infection is steady but high.
• hypoendemic d. — the rate of infection is steady and only a few animals are infected.
• immune complex d. — see immune complex disease.
• infectious d. — one caused by small living organisms including viruses, bacteria, fungi, protozoa and metazoan parasites. It may be contagious in origin, result from nosocomial infections or be due to endogenous microflora of the nose and throat, skin or bowel. See also communicable disease (above).
• manifestational d. classification — diseases arranged in the order of their clinical signs, epidemiological characteristics, necropsy lesions, e.g. sudden death diseases.
• mesoendemic d. — the disease occurs at an even rate and a moderate proportion of animals are infected.
• metabolic d. — see metabolic diseases.
• molecular d. — any disease in which the pathogenesis can be traced to a single, precise chemical alteration, usually of a protein, which is either abnormal in structure or present in reduced amounts. The corresponding defect in the DNA coding for the protein may also be known.
• multicausal d. — 1. a number of causative agents are needed to combine to cause the disease.
— 2. the same disease can be caused by a number of different agents.
• multifactorial d. — see multicausal disease (above).
• new d. — disease not previously recorded. May be variants on an existing disease, e.g. infectious bovine rhinotracheitis, or escapes from other species, e.g. the Marburg virus disease of humans.
• notifiable d. — a disease of which any occurrence is required by law to be notified to government authorities.
• organic d. — see organic disease.
• pandemic d. — a very widespread epidemic involving several countries or an entire continent.
• quarantinable d. — a disease which the law requires to be restricted in its spread by putting the affected animals, farms or properties on which it occurs in quarantine.
• reportable d. — see notifiable disease (above).
• d. reservoir — any animal or fomite in which an infectious disease agent is preserved in a viable state or multiplies and upon which it may depend for survival.
• secondary d. — 1. a disease subsequent to or a consequence of another disease or condition.
— 2. a condition due to introduction of incompatible, immunologically competent cells into a host rendered incapable of rejecting them by heavy exposure to ionizing radiation.
• self-limited d. — see self-limited.
• sex-limited d. — disease limited in its occurrence to one or other sex. See also sex-linked.
• sexually transmitted d. (STD) — a disease that can be acquired by sexual intercourse.
• slaughter d. control — see slaughter (2).
• sporadic d. — occurring singly and haphazardly; widely scattered; not epidemic or endemic. See also sporadic bovine encephalomyelitis, sporadic leukosis, sporadic lymphangitis.
• storage d. — see storage disease.
• d. syndrome — see syndrome.
• systemic d. — sufficiently widespread in the body to cause clinical signs referable to any organ or system, and in which localization of infection may occur in any organ.
• d. triangle — interaction between the host, the disease agent, and the environment.
• d. wastage — loss of income generated by production of milk, eggs, fiber, or loss of capital value because of diminution in the patient's value.
• wasting d. — any disease marked especially by progressive emaciation and weakness.
• zoonotic d. — disease capable of spread from animals to humans. See also zoonosis.

A definite deviation from the normal state characterized by a series of symptoms. Disease may be caused by developmental disturbances, genetic factors, metabolic factors, living agents, or physical, chemical, or radiant energy, or the cause may be unknown.

• Diseases and Infestations - disease: physical disorder with specific physiological cause and recognizable signs and symptoms
• Diagnostic Terminology - disease: particular disorder of organism with specific cause and recognizable symptoms
• Unnatural Deaths - disease: fatal wasting or impairment of body function

"Human disease" redirects here. For the Slayer song, see Soundtrack to the Apocalypse.

"Flare-up" redirects here. For the Transformers character, see Flareup (Transformers).

"Medical condition" redirects here. For the descriptive terminology using words such as "good", "serious", "critical", etc., see Medical state.

For other uses, see Disease (disambiguation).

A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs.^[1] It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune diseases. In humans, "disease" is often used more broadly to refer to any condition that causes pain, dysfunction, distress, social problems, or death to the person afflicted, or similar problems for those in contact with the person. In this broader sense, it sometimes includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes these may be considered distinguishable categories. Diseases usually affect people not only physically, but also emotionally, as contracting and living with many diseases can alter one's perspective on life, and their personality.

Death due to disease is called death by natural causes. There are four main types of disease: pathogenic disease, deficiency disease, hereditary disease, and physiological disease.

Diseases can also be classified as communicable and non-communicable disease.

Contents 1 Terminology 1.1 Disease 1.2 Illness 1.3 Disorder 1.4 Medical condition 1.5 Morbidity 2 Stages 3 Scope 4 Causes and transmissibility 5 Burdens of disease 6 Prevention 7 Treatments 8 Epidemiology 9 Social and culture 9.1 Language of disease 10 See also 11 References 12 External links

Terminology

In many cases, the terms disease, disorder, morbidity and illness are used interchangeably.^[2] In some situations, specific terms are considered preferable.

Disease

The term disease broadly refers to any condition that impairs normal function. Commonly, this term is used to refer specifically to infectious diseases, which are clinically evident diseases that result from the presence of pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular organisms, and aberrant proteins known as prions. An infection that does not and will not produce clinically evident impairment of normal functioning, such as the presence of the normal bacteria and yeasts in the gut, is not considered a disease; by contrast, an infection that is asymptomatic during its incubation period, but expected to produce symptoms later, is usually considered a disease. Non-infectious diseases are all other diseases, including most forms of cancer, heart disease, and genetic disease.

Illness

Illness and sickness are generally used as synonyms for disease.^[3] However, this term is occasionally used to refer specifically to the patient's personal experience of their disease.^[4][5] In this model, it is possible for a person to be diseased without being ill (to have an objectively definable, but asymptomatic, medical condition), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or medicalizes a non-disease situation in his or her life). Illness is often not due to infection but a collection of evolved responses, sickness behavior, by the body which aids the clearing of infection. Such aspects of illness can include lethargy, depression, anorexia, sleepiness, hyperalgesia, and inability to concentrate.^[6][7][8]

Disorder

In medicine, a disorder is a functional abnormality or disturbance. Medical disorders can be categorized into mental disorders, physical disorders, genetic disorders, emotional and behavioral disorders, and functional disorders.

The term disorder is often considered more value-neutral and less stigmatizing than the terms disease or illness, and therefore is preferred terminology in some circumstances. In mental health, the term mental disorder is used as a way of acknowledging the complex interaction of biological, social, and psychological factors in psychiatric conditions. However, the term disorder is also used in many other areas of medicine, primarily to identify physical disorders that are not caused by infectious organisms, such as metabolic disorders.

Medical condition

A medical condition is a broad term that includes all diseases and disorders. While the term medical condition generally includes mental illnesses, in some contexts the term is used specifically to denote any illness, injury, or disease except for mental illnesses. The Diagnostic and Statistical Manual of Mental Disorders (DSM), the widely used psychiatric manual that defines all mental disorders, uses the term general medical condition to refer to all diseases, illnesses, and injuries except for mental disorders.^[9] This usage is also commonly seen in the psychiatric literature. Some health insurance policies also define a medical condition as any illness, injury, or disease except for psychiatric illnesses.^[10]

As it is more value-neutral than terms like disease, the term medical condition is sometimes preferred by people with health issues that they do not consider to be deleterious. On the other hand, by emphasizing the medical nature of the condition, this term is sometimes rejected, such as by proponents of the autism rights movement.

The term medical condition is used as a synonym for medical state, where it describes a patient's current state, as seen from a medical standpoint. This usage is seen in statements that describe a patient as being "in critical condition", for example.

Morbidity

Morbidity (from Latin morbidus, meaning "sick, unhealthy") is a diseased state, disability, or poor health due to any cause.^[11] The term may be used to refer to the existence of any form of disease, or to the degree that the health condition affects the patient. Among severely ill patients, the level of morbidity is often measured by ICU scoring systems.

Comorbidity is the simultaneous presence of two medical conditions, such as a person with schizophrenia and substance abuse.

In epidemiology and actuarial science, the term morbidity rate can refer to either the incidence rate, or the prevalence of a disease or medical condition. This measure of sickness is contrasted with the mortality rate of a condition, which is the proportion of people dying during a given time interval.

Stages

In an infectious disease, the incubation period is the time between infection and the appearance of symptoms. The latency period is the time between infection and the ability of the disease to spread to another person, which may precede, follow, or be simultaneous with the appearance of symptoms. Some viruses also exhibit a dormant phase, called viral latency, in which the virus hides in the body in an inactive state. For example, varicella zoster virus causes chickenpox in the acute phase; after recovery from chickenpox, the virus may remain dormant in nerve cells for many years, and later cause herpes zoster (shingles).

A cure is the end of a medical condition or a treatment that is very likely to end it, while remission refers to the disappearance, possibly temporarily, of symptoms. Complete remission is the best possible outcome for incurable diseases.

A flare-up can refer to either the recurrence of symptoms or an onset of more severe symptoms.

A refractory disease is a disease that resists treatment, especially an individual case that resists treatment more than is normal for the specific disease in question.

Progressive disease is a disease whose typical natural course is the worsening of the disease until death, serious debility, or organ failure occurs. Slowly progressive diseases are also chronic diseases; many are also degenerative diseases. The opposite of progressive disease is stable disease or static disease: a medical condition that exists, but does not get better or worse.

Scope

A localized disease is one that affects only one part of the body, such as athlete's foot or an eye infection.

A disseminated disease has spread to other parts; with cancer, this is usually called metastatic disease.

A systemic disease is a disease that affects the entire body, such as influenza or high blood pressure.

Causes and transmissibility

Main article: Transmission (medicine)

Only some diseases such as influenza are contagious and commonly believed to be infectious. The micro-organisms that cause these diseases are known as pathogens and include varieties of bacteria, viruses, protozoa and fungi. Infectious diseases can be transmitted, e.g. by hand-to-mouth contact with infectious material on surfaces, by bites of insects or other carriers of the disease, and from contaminated water or food (often via faecal contamination), etc. In addition, there are sexually transmitted diseases. In some cases, micro-organisms that are not readily spread from person to person play a role, while other diseases can be prevented or ameliorated with appropriate nutrition or other lifestyle changes.

Some diseases, such as most (but not all) forms of cancer, heart disease and mental disorders, are non-infectious diseases. Many non-infectious diseases have a partly or completely genetic basis (see genetic disorder) and may thus be transmitted from one generation to another.

Social determinants of health are the social conditions in which people live which determine their health. Illnesses are generally related to social, economic, political, and environmental circumstances. Social determinants of health have been recognized by several health organizations such as the Public Health Agency of Canada and the World Health Organization to greatly influence collective and personal well-being. The World Health Organization's Social Determinants Council also recognizes Social determinants of health in poverty.

When the cause of a disease is poorly understood, societies tend to mythologize the disease or use it as a metaphor or symbol of whatever that culture considers to be evil. For example, until the bacterial cause of tuberculosis was discovered in 1882, experts variously ascribed the disease to heredity, a sedentary lifestyle, depressed mood, and overindulgence in sex, rich food, or alcohol—all the social ills of the time.^[12]

Burdens of disease

Disease burden is the impact of a health problem in an area measured by financial cost, mortality, morbidity, or other indicators.

There are several measures used to quantify the burden imposed by diseases on people. The years of potential life lost (YPLL) is a simple estimate of the number of years that a person's life was shortened due to a disease. For example, if a person dies at the age of 65 from a disease, and would probably have lived until age 80 without that disease, then that disease has caused a loss of 15 years of potential life. YPLL measurements do not account for how disabled a person is before dying, so the measurement treats a person who dies suddenly and a person who died at the same age after decades of illness as equivalent. In 2004, the World Health Organization calculated that 932 million years of potential life were lost to premature death.^[13]

The quality-adjusted life year (QALY) and disability-adjusted life year (DALY) metrics are similar, but take into account whether the person was healthy after diagnosis. In addition to the number of years lost due to premature death, these measurements add part of the years lost to being sick. Unlike YPLL, these measurements show the burden imposed on people who are very sick, but who live a normal lifespan. A disease that has high morbidity, but low mortality, will have a high DALY and a low YPLL. In 2004, the World Health Organization calculated that 1.5 billion disability-adjusted life years were lost to disease and injury.^[13]

Disease category	Percent of all YPLLs lost, worldwide[13]	Percent of all DALYs lost, worldwide[13]	Percent of all YPLLs lost, Europe[13]	Percent of all DALYs lost, Europe[13]	Percent of all YPLLs lost, US and Canada[13]	Percent of all DALYs lost, US and Canada[13]
Infectious and parasitic diseases, especially lower respiratory tract infections, diarrhea, AIDS, tuberculosis, and malaria	37%	26%	9%	6%	5%	3%
Neuropsychiatric conditions, e.g. depression	2%	13%	3%	19%	5%	28%
Injuries, especially motor vehicle accidents	14%	12%	18%	13%	18%	10%
Cardiovascular diseases, principally heart attacks and stroke	14%	10%	35%	23%	26%	14%
Premature birth and other perinatal deaths	11%	8%	4%	2%	3%	2%
Cancer	8%	5%	19%	11%	25%	13%

Prevention

Main article: Preventive medicine

Many diseases and disorders can be prevented through a variety of means. These include sanitation, proper nutrition, adequate exercise, vaccinations, and other self-care and public health measures.

Treatments

Main article: Therapy

Medical therapies or treatments are efforts to cure or improve a disease or other health problem. In the medical field, therapy is synonymous with the word "treatment". Among psychologists, the term may refer specifically to psychotherapy or "talk therapy". Common treatments include medications, surgery, medical devices, and self-care. Treatments may be provided by an organized health care system, or informally, by the patient or family members.

A prevention or preventive therapy is a way to avoid an injury, sickness, or disease in the first place. A treatment or cure is applied after a medical problem has already started. A treatment attempts to improve or remove a problem, but treatments may not produce permanent cures, especially in chronic diseases. Cures are a subset of treatments that reverse diseases completely or end medical problems permanently. Many diseases that cannot be completely cured are still treatable.Pain management (also called pain medicine) is that branch of medicine employing an interdisciplinary approach to the relief of pain and improvement in the quality of life of those living with pain^[14]

Treatment for medical emergencies must be provided promptly, often through an emergency department or, in less critical situations, through an urgent care facility.

Epidemiology

Main article: Epidemiology

Epidemiology is the study of the factors that cause or encourage diseases. Some diseases are more common in certain geographic areas, among people with certain genetic or socioeconomic characteristics, or at different times of the year.

Epidemiology is considered a cornerstone methodology of public health research, and is highly regarded in evidence-based medicine for identifying risk factors for disease. In the study of communicable and non-communicable diseases, the work of epidemiologists ranges from outbreak investigation to study design, data collection and analysis including the development of statistical models to test hypotheses and the documentation of results for submission to peer-reviewed journals. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic. Epidemiologists rely on a number of other scientific disciplines such as biology (to better understand disease processes), biostatistics (the current raw information available), Geographic Information Science (to store data and map disease patterns) and social sciencedisciplines (to better understand proximate and distal risk factors).

In studying diseases, epidemiology faces the challenge of defining them. Especially for poorly understood diseases, different groups might use significantly different definitions. Without an agreed-upon definition, different researchers will find very different numbers of cases and characteristics of the disease.^[15]

Social and culture

Obesity was a status symbol in Renaissance culture: "The Tuscan General Alessandro del Borro", attributed to Andrea Sacchi, 1645.^[16] It is now generally regarded as a disease.

How society responds to disease is the subject of medical sociology.

A condition may be considered to be a disease in some cultures or eras but not in others. For example, obesity can represent wealth and abundance, and is a status symbol in famine-prone areas and some places hard-hit by HIV/AIDS.^[17] Epilepsy is considered a sign of spiritual gifts among the Hmong people.^[18]

Sickness confers the social legitimization of certain benefits, such as illness benefits, work avoidance, and being looked after by others. The person who is sick takes on a social role called the sick role. A person who responds to a dreaded disease, such as cancer, in a culturally acceptable fashion may be publicly and privately honored with higher social status.^[19] In return for these benefits, the sick person is obligated to seek treatment and work to become well once more. As a comparison, consider pregnancy, which is not usually interpreted as a disease or sickness, even if the mother and baby may both benefit from medical care.

Most religions grant exceptions from religious duties to people who are sick. For example, one whose life would be endangered by fasting on Yom Kippur or during Ramadan is exempted from the requirement, or even forbidden from participating. People who are sick are also exempted from social duties. For example, ill health is the only socially acceptable reason for an American to refuse an invitation to the White House.^[20]

The identification of a condition as a disease, rather than as simply a variation of human structure or function, can have significant social or economic implications. The controversial recognitions as diseases of post-traumatic stress disorder, also known as "Soldier's heart", "shell shock", and "combat fatigue;" repetitive motion injury or repetitive stress injury (RSI); and Gulf War syndrome has had a number of positive and negative effects on the financial and other responsibilities of governments, corporations and institutions towards individuals, as well as on the individuals themselves. The social implication of viewing aging as a disease could be profound, though this classification is not yet widespread. Lepers were a group of afflicted individuals who were historically shunned and the term "leper" still evokes social stigma. Fear of disease can still be a widespread social phenomenon, though not all diseases evoke extreme social stigma.

Social standing and economic status affect health. Diseases of poverty are diseases that are associated with poverty and low social status; diseases of affluence are diseases that are associated with high social and economic status. Which diseases are associated with which states varies according to time, place, and technology. Some diseases, such as diabetes mellitus, may be associated with both poverty (poor food choices) and affluence (long lifespans and sedentary lifestyles), through different mechanisms. The term diseases of civilization describes diseases that are more common among older people. For example, cancer is far more common in societies in which most members live until they reach the age of 80 than in societies in which most members die before they reach the age of 50.

Language of disease

An illness narrative is a way of organizing a medical experience into a coherent story that illustrates the experience.

People use metaphors to make sense of their experiences with disease. The metaphors move disease from an objective thing that exists to an affective experience. The most popular metaphors draw on military concepts: Disease is an enemy that must be feared, fought, battled, and routed. The patient or the physician is a warrior, rather than a passive victim or bystander. The agents of communicable diseases are invaders; non-communicable diseases constitute internal insurrection. Because the threat is urgent, perhaps a matter of life and death, unthinkably radical, even oppressive, measures are society's and the patient's moral duty as they courageously mobilize to struggle against destruction. The War on Cancer is an example.^[21]

Another class of metaphors describes the experience of illness as a journey: The person travels to or from a place of disease, and changes himself, discovers new information, or increases his experience along the way. He may travel "on the road to recovery" or make changes to "get on the right track".^[21] Some are explicitly immigration-themed: the patient has been exiled from the home territory of health to the land of the ill, changing identity and relationships in the process.^[22]

Some metaphors are disease-specific. Slavery is a common metaphor for addictions: The alcoholic is enslaved by drink, and the smoker is captive to nicotine. Some cancer patients treat the loss of their hair from chemotherapy as a metonymy or metaphor for all the losses caused by the disease.^[21]

Some diseases are used as metaphors for social ills: "Cancer" is a common description for anything that is endemic and destructive in society, such as poverty, injustice, or racism. AIDS was seen as a divine judgment for moral decadence, and only by purging itself from the "pollution" of the "invader" could society become healthy again.^[21] Authors in the 19th century commonly used tuberculosis as a symbol and a metaphor for transcendence. Victims of the disease were portrayed in literature as having risen above daily life to become ephemeral objects of spiritual or artistic achievement. In the 20th century, the same disease became the emblem of poverty, squalor, and other social problems.^[22]

See also

• List of acronyms on diseases and disorders
• List of cutaneous conditions
• Lists of diseases
• Airborne disease, a disease that spreads through the air
• Contagious disease, a subset of infectious diseases
• Cryptogenic disease, a disease whose cause is currently unknown
• Disseminated disease, a disease that is spread throughout the body
• Environmental disease
• Lifestyle disease, a disease caused largely by lifestyle choices
• Localized disease, a disease affecting one body part or area
• Non-communicable disease, a disease that can't be spread between people
• Organic disease
• Progressive disease, a disease that gets worse over time
• Rare disease, a disease that affects very few people
• Systemic disease, a disease affecting the whole body
• Plant pathology
• Sociology of health and illness
• List of mystery diseases

References

1. ^ "Disease" at Dorland's Medical Dictionary
2. ^ "Mental Illness—Glossary". US National Institute of Mental Health. http://science.education.nih.gov/supplements/nih5/mental/other/glossary.htm. Retrieved 2010-04-18. 
3. ^ "Illness" at Dorland's Medical Dictionary
4. ^ Emson HE (April 1987). "Health, disease and illness: matters for definition". CMAJ 136 (8): 811–3. PMC 1492114. PMID 3567788. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1492114. 
5. ^ McWhinney IR (April 1987). "Health and disease: problems of definition". CMAJ 136 (8): 815. PMC 1492121. PMID 3567791. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1492121. 
6. ^ Hart BL (1988). "Biological basis of the behavior of sick animals". Neurosci Biobehav Rev 12 (2): 123–137. doi:10.1016/S0149-7634(88)80004-6. PMID 3050629. 
7. ^ Johnson R (2002). "The concept of sickness behavior: a brief chronological account of four key discoveries". Veterinary Immunology and Immunopathology 87 (3-4): 443–450. doi:10.1016/S0165-2427(02)00069-7. PMID 12072271. 
8. ^ Kelley KW, Bluthe RM, Dantzer R, Zhou JH, Shen WH, Johnson RW, Broussard SR (2003). "Cytokine-induced sickness behavior". Brain Behav Immun 17 (Suppl 1): S112–118. doi:10.1016/S0889-1591(02)00077-6. PMID 12615196. 
9. ^ American Psychiatric Association. Task Force on DSM-IV (2000). Diagnostic and statistical manual of mental disorders (4th ed.). Washington, DC: American Psychiatric Association. ISBN 978-0-89042-025-6. 
10. ^ "Expat Insurance Glossary by The Insurance Page". http://www.theinsurancepage.co.uk/expat-insurance-glossary.html. Retrieved 2008-11-20. 
11. ^ Dorland's Medical Dictionary: morbidity, Dorland's Medical Dictionary, MerckSource
12. ^ Olson, James Stuart (2002). Bathsheba's breast: women, cancer & history. Baltimore: The Johns Hopkins University Press. pp. 168–170. ISBN 0-8018-6936-6. 
13. ^ ^{a b c d e f g h} :"Disease and injury regional estimates for 2004". World Health Organization. http://www.who.int/healthinfo/global_burden_disease/estimates_regional/en/index.html. 

    Standard DALYs (3% discounting, age weights).

    DALY spreadsheet

    YLL spreadsheet.

14. ^ Hardy, Paul A.; Hardy, Paul A. J. (1997). Chronic Pain Management: The Essentials. Cambridge University Press. p. 10. ISBN 978-1-900151-85-6. OCLC 36881282. http://books.google.com/books?id=EtZ-4eb_aDUC&pg=PA10&lpg=PA10. 
15. ^ Tuller, David (4 March 2011). "Defining an illness is fodder for debate". The New York Times. http://www.nytimes.com/2011/03/08/health/research/08fatigue.html. 
16. ^ Carol Gerten-Jackson. "The Tuscan General Alessandro del Borro". http://www.cab.u-szeged.hu/cgfa/m/m-12.htm. 
17. ^ Haslam DW, James WP (2005). "Obesity". Lancet 366 (9492): 1197–209. doi:10.1016/S0140-6736(05)67483-1. PMID 16198769. 
18. ^ Fadiman, Anne (1997). The spirit catches you and you fall down: a Hmong child, her American doctors, and the collision of two cultures. New York: Farrar, Straus, and Giroux. ISBN 0-374-52564-1. 
19. ^ Sulik, Gayle (2010). Pink Ribbon Blues: How Breast Cancer Culture Undermines Women's Health. New York: Oxford University Press. ISBN 0-19-974045-3. 
20. ^ Martin, Judith (2005). Miss Manners' Guide to Excruciatingly Correct Behavior. New York: W.W. Norton & Co. pp. 703. ISBN 978-0-393-05874-1. OCLC 57549405. 
21. ^ ^{a b c d} Gwyn, Richard (1999). "10". In Cameron, Lynne; Low, Graham. Researching and applying metaphor. Cambridge, England: Cambridge University Press. ISBN 978-0-521-64964-3. OCLC 40881885. 
22. ^ ^{a b} Diedrich, Lisa (2007). Treatments: language, politics, and the culture of illness. Minneapolis: University of Minnesota Press. pp. 8, 29. ISBN 978-0-8166-4697-5. OCLC 601862594. 

External links

Wikimedia Commons has media related to: diseases

Wikiquote has a collection of quotations related to: Disease

Look up disease in Wiktionary, the free dictionary.

• Health Topics, MedlinePlus descriptions of most diseases, with access to current research articles.
• OMIM Comprehensive information on genes that cause disease at Online Mendelian Inheritance in Man
• CTD The Comparative Toxicogenomics Database is a scientific resource connecting chemicals, genes, and human diseases.
• NLM Comprehensive database from the US National Library of Medicine
• Health Topics A-Z, fact sheets about many common diseases at Centers for Disease Control
• The Merck Manual containing detailed description of most diseases
• Report: The global burden of disease from World Health Organization (WHO), 2004
• Free online health-risk assessment by Your Disease Risk at Washington University in St Louis

v t e Medicine: Pathology Principles of pathology Disease/Medical condition Infection Neoplasia Hemodynamics Ischemia Inflammation Wound healing Cell death Necrosis Liquefactive necrosis Coagulative necrosis Caseous necrosis Fat necrosis Apoptosis Pyknosis Karyorrhexis Karyolysis Cellular adaptation Atrophy Hypertrophy Hyperplasia Dysplasia Metaplasia Squamous Glandular Accumulations: pigment Hemosiderin Lipochrome/Lipofuscin Melanin Steatosis Anatomical pathology Surgical pathology Cytopathology Autopsy Molecular pathology Forensic pathology Dental pathology Gross examination Histopathology Immunohistochemistry Electron microscopy Immunofluorescence Fluorescent in situ hybridization Clinical pathology Clinical chemistry Hematopathology Transfusion medicine Medical microbiology Diagnostic immunology Immunopathology Enzyme assay Mass spectrometry Chromatography Flow cytometry Blood bank Microbiological culture Serology Specific conditions Myocardial infarction

v t e Pathology: Medical conditions and ICD code (Disease / Disorder / Illness, Syndrome / Sequence, Symptom / Sign, Injury, etc.) (A/B, 001–139) Infectious disease/Infection: Bacterial disease (G+, G-) · Virus disease · Parasitic disease (Protozoan infection, Helminthiasis, Ectoparasitic infestation) · Mycosis · Zoonosis (C/D, 140–239 & 279–289) Cancer (C00–D48, 140–239) Tumor Myeloid hematologic (D50–D77, 280–289) Anemia · Coagulopathy Lymphoid immune (D80–D89, 279) Immunodeficiency · Immunoproliferative disorder · Hypersensitivity (E, 240–278) Endocrine disease · Nutrition disorder · Inborn error of metabolism (F, 290–319) Mental disorder (G, 320–359) Nervous system disease (CNS, PNS) · Neuromuscular disease (H, 360–389) Eye disease · Ear disease (I, 390–459) Cardiovascular disease (Heart disease, Vascular disease) (J, 460–519) Respiratory disease (Obstructive lung disease, Restrictive lung disease, Pneumonia) (K, 520–579) Stomatognathic disease (Tooth disease) · Digestive disease (Esophageal, Stomach, Enteropathy, Liver, Pancreatic) (L, 680–709) Skin disease · skin appendages (Nail disease, Hair disease, Sweat gland disease) (M, 710–739) Musculoskeletal disorders: Myopathy · Arthropathy · Osteochondropathy (Osteopathy, Chondropathy) (N, 580–629) Urologic disease (Nephropathy, Urinary bladder disease) · Male genital disease · Breast disease · Female genital disease (O, 630–679) Complications of pregnancy · Obstetric labor complication · Puerperal disorder (P, 760–779) Fetal disease (Q, 740–759) Congenital disorder (Congenital abnormality) (R, 780–799) Syndromes · Medical signs (Eponymous) (S/T, 800–999) Bone fracture · Joint dislocation · Sprain · Strain · Subluxation · Head injury · Chest trauma · Poisoning

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Dansk (Danish)
n. - sygdom, plantesygdom

Nederlands (Dutch)
ziekte, wantoestand

Français (French)
n. - maladie, affection, (Bot, Vét) maladie, (fig) maladie, mal

Deutsch (German)
n. - Krankheit

Ελληνική (Greek)
n. - αρρώστια, νόσος, νόσημα, πάθηση, ασθένεια, μάστιγα

Italiano (Italian)
malattia

Português (Portuguese)
n. - doença (f), enfermidade (f)

Русский (Russian)
болезнь

Español (Spanish)
n. - enfermedad, dolencia, mal

Svenska (Swedish)
n. - sjukdom

中文（简体）(Chinese (Simplified))
疾病, 弊病

中文（繁體）(Chinese (Traditional))
n. - 疾病, 弊病

한국어 (Korean)
n. - 질병, (의식 등의) 건전하지 못한 상태, (술의) 변질

日本語 (Japanese)
n. - 病気
v. - 病気にかからせる

idioms:

• Alzheimer's disease    アルツハイマー病, 老年性痴呆
• Hodgkin's disease    ホジキン病
• Legionnaire's disease    在郷軍人病
• Parkinson's disease    パーキンソン病
• wasting disease    消耗病

العربيه (Arabic)
‏(الاسم) مرض, عله‏

עברית (Hebrew)
n. - ‮מחלה‬

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