Infectious disease

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(in′fek·shəs di′zēz)

(medicine) Any disease caused by invasion by a pathogen which subsequently grows and multiplies in the body.

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Concept

The history of the human species, it has been said, is the history of infectious disease. Over the centuries, humans have been exposed to a vast amount and array of contagious conditions, including the Black Death and other forms of plague, typhoid fever, cholera, malaria, influenza, and the acquired immunodeficiency syndrome, or AIDS. Only in the past few hundred years have scientists begun to have any sort of accurate idea concerning the origin of such diseases, through the action of microorganisms and other parasites. Such understanding has led to the development of vaccines and methods of inoculation, yet even before they made these great strides in medicine, humans had an unseen protector: their own immune systems.

How It Works

Infection and Immunity

There are two basic types of disease: ones that are infectious, or extrinsic, meaning that they are contagious or communicable and can be spread by contact between people, and ones that are intrinsic, or not infectious. Diseases in general and noninfectious diseases in particular are discussed in essays devoted to those subjects. So, too, is infection itself, a subject separate from infectious diseases: a person can get an infection, such as tetanus or salmonella, without necessarily having a disease that can be passed on through contact with others in the same way that colds, malaria, or syphilis is spread.

The background on scientists' progressive understanding of the microorganisms that cause disease and the means of fighting these microorganisms are discussed in Infection. Among the leading figures in that history were the French chemist and microbiologist Louis Pasteur (1822-1895) and the German bacteriologist Robert Koch (1843-1910), who contributed greatly to what is known today as germ theory—the idea that infection and infectious diseases are brought about by microorganisms. In most cases, the organisms are too small to be seen with the naked eye. They include varieties of amoeba and worm, discussed in the essay Parasites and Parasitology, as well as viruses and some forms of bacteria and fungi, which together are known as pathogens, or disease-carrying parasites. Other terms related to infectious diseases, their agents, and the prevention and study of them are defined in the essay Infection.

Immune Mechanisms

The human body has numerous mechanisms for protecting itself from infectious disease, the first line of defense being the skin. Skin shields us all the time from unseen attackers and generally is able to prevent pathogens from entering the body; however, any break in the skin, such as a cut or scrape, provides an opening for microorganisms to invade the body. Germs that normally would be prevented from entering the body are able to invade the bloodstream through such openings. This is why it is so very important, in any situation involving potential contact with infection, to protect the skin. With the advent of AIDS, doctors and members of other professions who are likely to touch people carrying diseases—including officers arresting addicts or prostitutes—are much more likely to do their work wearing heavy plastic gloves.

Suppose that a microorganism makes it through the barrier of skin, thanks to a cut or other opening; if so, the body puts into action a second defensive mechanism, the immune system. This system is a network of organs, glands, and tissues that protects the body from foreign substances. Without a properly functioning immune system, a person could die simply by walking out the door in the morning and coming into contact with an airborne infectant. Even in relatively healthy people, the immune system may be unable to react adequately to an invasion of microorganisms. In such cases, disease develops.

Transmission of Diseases

Infectious diseases, by definition, are transmitted easily from one person to another. We have all been told, for instance, not to drink after someone who has a cold. On a much more serious level, persons who are sexually active or potentially sexually active, but not settled in a monogamous (one-partner) relationship, are advised to avoid unprotected sexual contact so as not to contract AIDS or some other sexually transmitted disease (STD). In these and many other cases, microorganisms travel from the carrier of the disease to the uninfected person. (Actually, in the case of AIDS, the pathogen is a virus, which is not, strictly speaking, an organism or even a living thing; however, viruses usually are lumped in with bacteria, amoeba, and some fungi as microorganisms.)

Pathogens can be spread by many methods other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. For instance, any time a person with an infection coughs or sneezes, they may be transmitting illness. This is how diseases such as measles and tuberculosis are passed from person to person. AIDS and various STDs, as well as many other conditions, such as hepatitis, are transferred when one person comes into contact with the bodily fluids of another. This is the case not only with sexual intercourse but also with blood transfusions and any number of other interactions, including possibly drinking after someone. (Contrary to rumors that circulated in the early 1980s, when AIDS first made itself known, that particular syndrome cannot be transferred by saliva, but the common cold and other viral infections can be.)

Cholera, caused by a bacterium found in dirty wells and rivers from India to England (in the 1800s, at least), is an example of a waterborne disease. Many foodborne pathogens tend to bring about what would be more commonly thought of as an illness than a disease, since in everyday language the latter term implies a long-term affliction, whereas food poisoning usually lasts for a week or so. (Still, some forms of food poisoning can be fatal.) Bacterial contamination may occur when food is not cooked thoroughly, is left unrefrigerated, is prepared by an infected food handler, or otherwise is handled in an unsanitary or improper fashion. (The case of Typhoid Mary, discussed near the conclusion of this essay, is an extreme example of this form of transmission.)

Additionally, diseases may be transferred by vectors—animals (usually insects) that carry microorganisms from one person to another. Vectors may spread a disease either by mechanical or by biological means. Mechanical transmission occurs, for example, when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission takes place when an insect bites a person and takes infected blood into its own system. Once inside the insect's gut, the disease-causing organisms may reproduce, increasing the number of parasites that can be transmitted to the next victim. This is how the Anopheles mosquito vector, for instance, transfers malaria.

Real-Life Applications

A Tour of Diseases

The range of infectious diseases, from conditions that merely cause discomfort to those that bring about death, is truly staggering. Some have brought about vast epidemics that have wiped out huge populations, and many have changed the course of history, while others are hardly known to anyone outside the ranks of epidemiologists and the victims of the disease. Some, such as smallpox, have been eradicated or largely eradicated through inoculation campaigns, while others, most notably AIDS, continue to elude efforts to defeat them.

Diseases can be classified according to the systems or body parts affected. Some of those systems and parts, with examples of diseases relating to each, include the following.

• Upper respiratory tract: common cold, sinusitis, croup
• Lower respiratory tract: pneumonia, bronchitis
• Cardiovascular system: rheumatic fever
• Central nervous system: meningitis, encephalitis
• Genitourinary tract: sexually transmitted diseases (i.e., venereal diseases, such as syphilis, gonorrhea, and the herpes simplex viral infection)
• Gastrointestinal tract: cholera, salmonella, hepatitis
• Bones and joints: septic arthritis
• Skin: warts, candida
• Eyes: conjunctivitis (pink eye)

Another way to classify diseases is according to the types of organism that cause them: bacteria, viruses, or other forms of parasite, particularly worms, amoeba, and insects. The first two groups are discussed in further detail within Infection and the other varieties of parasite in Parasites and Parasitology.

Bacterial infections include anthrax, botulism, tetanus (lockjaw), leprosy, tuberculosis, diphtheria, whooping cough, plague, and a variety of pneumococcal, staphylococcal, and streptococcal illnesses. Among viral illnesses and diseases are the common cold, influenza, infectious mononucleosis, smallpox, chicken pox, measles, mumps, rubella (or German measles), yellow fever, poliomyelitis (i.e., polio), rabies, herpes simplex, and AIDS. Diseases related to other varieties of parasite include malaria, Rocky Mountain spotted fever, trichinosis, scabies, and river blindness. Nonmicroscopic parasites, particularly such worms as hookworm and pin-worm, bring about disease-like forms of parasitic infestation within the body.

Plagues

From earliest times infectious diseases have wreaked havoc on the human species, and this was particularly so with the various plagues that struck Europe in ancient and medieval times. As noted in Infection, a plague in the fifth century B.C. helped bring an end to the golden age of Greek civilization. A thousand years later, another plague befell Greece, which by then dominated what remained of the Roman Empire. Based in Byzantium (Constantinople) this realm became known to history as the Byzantine (Eastern Roman), Empire, though its citizens saw themselves simply as "Romans" and thus as the inheritors of Roman civilization. Italy itself had fallen under the control of nomadic invaders, the Visigoths, but Emperor Justinian I (483-565) undertook a vast and costly campaign to wrest control of the Italian peninsula from the barbarians. Had he succeeded, the entire course of medieval history in Western Europe might have been different; he did not, largely because of a plague that swept Constantinople in 541.

Through a series of interconnected events, the plague permanently weakened Byzantium and left the Mediterranean world ripe for conquest by a new power: Islam. Both directly and indirectly, the plague of 541 served to divide Eastern and Western Europe. Not only was the Roman Empire never truly reunited, meaning that the two halves of the continent grew increasingly separate, but the rise of Islam made possible the Crusades (1095-1291). The latter sowed further discord between the East and the West, owing to the fact that Western European crusaders overran Byzantium and incited trouble between the Byzantines and Arabs. Ultimately, the split between Eastern and Western Europe, which became particularly pronounced during the years of Communism and the Iron Curtain (1945-1990), can be traced to the plague of 541.

The Black Death

The Byzantine plagues (there were several, occurring at intervals of a few generations), killed millions of people, yet for sheer scope of destruction—and, perhaps, historical impact—they were dwarfed by the plague that devastated Europe in the years 1347-1351. This one became known as the Plague (with a capital P) or by another name that gave some hint of the terror that was as much a part of the epidemic as the ghastly physical symptoms it brought on: the Black Death.

It began in Asia and quickly made its way to the shores of the Black Sea, where it erupted in September 1346. The first outbreak in Western Europe occurred 13 months later, in October 1347, at the Sicilian port of Messina, from whence it was an easy jump to the Italian mainland. By the following April all of Italy was infected; meanwhile, the Plague had reached Paris in January 1348, and within a year, 800 people a daywere dying in that city alone. Quickly it penetrated the entire European continent and beyond, from North Africa to Scandinavia and from England to the hinterlands of Russia. By 1351 it hadspread so far and wide that sailors arriving in Greenland found its ports deserted.

The only merciful thing about the Black Death was that death came quickly. Victims typically died within four days—a hundred hours of agony. If they caught a strain of bubonic plague, their lymph glands swelled; if it was pneumonicplague, the lungs succumbed first. Either way, as the end approached, the victim turned purplish-black from respiratory failure—hence the name Black Death.

Social Impact of the Plague

Lacking any modern concept of what causes disease, people looked for spiritual explanations. Some believed that the world was coming to an end, while others joined sects of flagellants, religious enthusiasts who wandered the countryside, beating themselves with lashes as a way of doing penance. The flagellants were tied closely tied to a rising trend toward anti-Semitism: searching for someone to blame, Europeans found a convenient scapegoat in the Jews, who, they claimed, had started the Plague by poisoning the wells of Europe.

The Black Death aptly illustrates how infectious diseases can have an impact on history in ways both big and small. In just five years the disease killed about 30% of Europe's population, which had been 100 million in 1300 but which would not reach that level again until 1500. All over the continent, farms were emptied and villages abandoned, leading to scarcity and higher prices. In the short run, these economic conditions spurred peasant revolts, but in the long run, the shortage of workers brought about higher wages and contributed to the emergence of the working and middle classes. Neither popes nor priests, neither kings nor noblemen, were any more equipped than the common people to confront the fearsome disease, and this, too, helped provoke the rise of competing classes and new centers of power in European society.

The Etiology of the Plague

The Black Death, in short, may be regarded as the beginning of the end of the Middle Ages—a hideously painful event that nevertheless carried positive consequences, which might hardly have been achieved without it. The irony was that the force at the center of all this devastation and change was too small to be seen by the naked eye. Although the disease was carried by rats, the cause of the Black Death was actually a bacillus known today as Pastuerella pestis or Yersinia pestis, which uses fleas as a vector. Modern medicines such as streptomycin, a variety of antibiotic developed after World War II, would have stopped the Plague, but such concepts were a long time in coming. Although the worst phase of the epidemic ended in 1351, it continued to spread, reaching Moscow by 1353; the next five centuries saw occasional outbreaks of the disease. As late as 1894 a strain of plague killed more than six million people in Asia over the course of 14 years.

The Changing Face of Disease

The many biblical passages dealing with leprosy illustrate the role that infectious disease has played in human life from the earliest times. The fact that leprosy causes the victim's skin to turn ghostly white and brings about a gradual withering away of body parts must certainly have seemed like a curse from God. In fact, leprosy, also known as Hansen disease, is caused by the bacillus Mycobacterium leprae, and despite the many fears throughout the ages associated with touching lepers, it is not very contagious. A scene in the 1973 blockbuster Papillon illustrates this fact. The title character, a prison escapee played by Steve McQueen, takes a drag from a cigar offered to him by a leper, who then asks him if he knew that leprosy is not contagious. Papillon says no, indicating that he simply intended to build a sense of shared risk with someone who he hoped would aid his escape.

The example of leprosy shows something about the many curiosities involved in diseases and their study: for example, the fact that a disease can be infectious without being significantly contagious. Leprosy is by definition infectious, inasmuch as it is caused by a pathogen known as Mycobacterium leprae, but the latter is unusual for a number of reasons, including the fact that it is extremely slow in dividing, unlike most bacteria. After years of study, researchers are still not clear as to how leprosy is transmitted, and many believe that genetics may play a role. Thanks to increased understanding of the disease, the stigma that used to go with leprosy—including the reference to people with the disease as "lepers"—has largely been lifted. Yet places such as the leprosy facilities at Carville, Louisiana, and Molokai, Hawaii, continued to exist for many years, if only because the disfigurement associated with the disease influenced the separation of leprosy sufferers from the rest of society. In 1998, with only about 6,000 victims of the disease left in the entire country, the federal government closed the facilities at Carville and Molokai.

Leprosy remains a threat, with some two million cases of the disease worldwide, primarily in nations of Asia, Africa, and Latin America that are both underdeveloped and located in tropical zones. It has, however, ceased to be the worldwide danger that it once was, and as such it joins ranks with numerous other afflictions that formerly held all of humankind in the grip of terror. For example, tuberculosis, caused by a bacillus that attacks the lungs, afflicted a huge population in the nineteenth century, bringing an end to the careers of figures that ranged from the great English poet John Keats to the American gunslinger Doc Holliday. Holliday, in fact, traveled to Tombstone, Arizona, where he and Wyatt Earp participated in the infamous shootout at the O.K. Corral, because he thought the climate would help his condition. Their story has been portrayed in countless films; for example, in Tombstone (1993), Val Kilmer gives an extremely convincing portrayal of the debilitating effects that Holliday's tuberculosis (aggravated by his lifestyle) must have had on him. Today, tuberculosis is not nearly the scourge that it once was, though it remains a problem, particularly because of patients' increasing resistance to the antibiotics used to treat it. (See Infection for more about antibiotics.)

Vaccination and Continuing Threats

When Europeans invaded the lands of Native Americans, they brought with them a host of microorganisms to which they had developed an immunity but to which the Indians were completely vulnerable. Although Europeans and their descendants had developed immunities to various diseases, thanks to generations of exposure to pathogens, they and the rest of the world remained vulnerable to a host of contagious disease, including cholera, smallpox, chicken pox, measles, mumps, yellow fever, polio, malaria, and many others. Today, vaccines have virtually eradicated many of these contagious diseases and keep others at bay. (Anyone who has ever had a cholera vaccine, which causes the patient's body to become miserably sore, achy, and tender for about 48 hours, has some idea of just how awful the disease itself must be.) Polio, which once posed an enormous threat to American children and crippled one of America's greatest leaders, President Franklin D. Roosevelt, is an artifact of history, thanks to vaccines developed after World War II.

Yet some killers never really die. For instance, malaria, caused by a protozoan parasitic genus known as Plasmodium and spread by mosquito biological vectors, infects from 300 to 500 million people annually and kills up to 2.7 million people every year. Although the substance known as quinine showed some promise as a treatment during most of the nineteenth and twentieth centuries, Plasmodium has become increasingly resistant to it. In the search for a cure for what has been called "the most devastating disease in history," some 100,000 drugs have been tested.

Some Other Killers

The twentieth century saw its own version of the Plague, in the form of the 1918-1920 influenza epidemic. Carried to all corners of the globe by soldiers returning from World War I, "the Influenza," as it came to be known (again with a capital letter to distinguish it as the greatest outbreak of a particular disease), killed 20 million people—more than the war itself. Then there is the greatest epidemic of the latter part of the twentieth century and the early twenty-first century: AIDS. This disease is linked to the human immunodeficiency virus (HIV), a retrovirus (see Infection for an explanation of retrovirus) that causes a gradual breakdown of the victim's immune system.

People do not die of AIDS per se but of the illnesses—particularly pneumonia or Kaposi's sarcoma, a cancer of the tissues—to which AIDS makes them susceptible. The disease is transmitted primarily by sexual contact and intravenous drug use. A smaller number of particularly tragic cases result from no actions on the part of the victim, who in this case is either the recipient of infected blood or the child of a mother with AIDS. Since the disease first came to public attention in 1981, 21.8 million people worldwide (and about 750,000 in the United States) have died from it. The vast majority of deaths have been in sub-Saharan Africa, and 90% of all AIDS cases are in developing countries. Worldwide, approximately 36.1 million people have either HIV or AIDS. (For more about AIDS, see Immunity and Immunology.)

The Ebola Virus

AIDS was not the only infectious condition to come out of central Africa and terrorize the world in the late twentieth century. Beginning in about 1975, numerous viruses, previously unknown and terrifyingly lethal, emerged from tropical regions of Africa, South America, and Asia. So great was the rise of new infectious diseases that some epidemiologists believed this was tied with economic development: as humans cultivated previously undeveloped lands and delved into more isolated parts of the world, they might be exposing new viruses.

Few of these inspired as much terror as the Ebola virus, and the fear is understandable, given the effects of the disease. Three to nine days after the illness enters the body, the victim begins to experience fever and other flu-like symptoms, sudden exhaustion, sore throat, muscle pain, and headache. Vomiting and diarrhea soon follow, and the vomit and stools are black with blood. Soon hemorrhaging occurs, with blood flowing from the nose, ears, and even the eyes. Internal organs begin to liquefy, and within three weeks of contracting the virus, the victim is usually dead.

An almost unbelievably hideous condition, Ebola might seem at first glance a great deal like the Black Death. Why, then, has it not ravaged whole populations the way the Plague did? It is certainly not because scientists have a cure for Ebola; the best doctors can hope to do, if they detect the disease early enough, is to provide supportive care, such as blood transfusions, that may save the patient's life. Yet even the worst outbreaks of the disease have not occurred on anything like the scale of the Plague: the worst known outbreak of Ebola, in Uganda in 2000-2001, killed 425 people.

Part of the reason Ebola is not capable of spreading rapidly is, ironically, because it is such an efficient killer: it kills its human victims before they have a chance to spread it to many other victims. Other than nonfatal incidents in laboratories in the United States, England, and Italy, as well as one case in a monkey export facility in the Philippines—various primates are carriers—all Ebola cases and outbreaks have been in Africa, primarily in Zaire (now Democratic Republic of the Congo), Sudan, and Gabon. Many times, local conditions, situations, and practices have exacerbated the spread of the disease. For example, in 1996, a group of people in Gabon found a dead chimpanzee in the forest and ate it; as a result, 37 people died. The Uganda outbreak became much worse than it might have been because locals, lacking education as to antiseptic procedures, failed to take proper precautions. Many died as a result of attending funerals of Ebola victims at which bodies were not disposed of properly.

Typhoid Mary

Sometimes a single person can be a walking epidemic, as in the case of the Irish cook Mary Mallon (1869-1938), better known as "Typhoid Mary." Mallon was an example of the fact that some people, because of genetic characteristics or other specifics, can act as carriers of a disease without ever contracting it themselves. Even though Typhoid Mary had Salmonella typhosa bacteria in her system, she did not get sick; still, she was highly contagious, and her profession as cook made her particularly dangerous. At least three deaths and 53 cases of typhoid fever were linked directly to her, with thousands of other probable cases of infection indirectly caused by this human vector.

Part of what made her so notorious—hence her nickname, given to her by the press—was the fact that Mallon did not seem to care how many people she infected. In the first decade of the twentieth century, authorities tracked her down as the cause of, or at least a contributing factor in, an outbreak of typhoid in the New York City area. Instead of cooperating with officials, Mallon repeatedly escaped before being caught and confined to Riverside Hospital on New York's North Brother Island in 1910. She served three years in isolation there before her release, after which she promptly went back to work as a cook—despite explicit orders not to do so. It was this (and an outbreak of typhoid fever at her place of work, which happened to be a hospital) that earned her the nickname by which she became known to history. She was caught again in 1915 and spent the remainder of her life on North Brother Island.

The Threat of Biological Warfare

Infinitely more despicable than Typhoid Mary are terrorists and rogue nations that would willingly unleash infectious disease on large, unsuspecting civilian populations. One such pathogen is Bacillus anthracis, the cause of anthrax, a deadly bacterial disease of cattle and other grazing animals. Under the right circumstances, anthrax can kill a human in about 36 hours, though a number of antibiotic treatments are effective in the early stages of the disease.

During the late twentieth century, the United States and Soviet Union experimented with the use of anthrax in biological warfare, and an accidental release of anthrax spores at a Soviet lab in 1979 led to some 68 deaths. Following the September 11, 2001, terrorist attacks on the World Trade Center in New York City and on the Pentagon, a series of letters containing anthrax spores showed up around the United States, and exposure to the disease led to a handful of deaths. Although the attacks were linked initially to Osama bin Laden and his al-Qaeda organization, authorities increasingly began to suspect that home-grown terrorists were simply exploiting the September 11 attacks as cover for their own deeds.

Still, there was little doubt that bin Laden, the Iraqi dictator Saddam Hussein, or North Korea's ruling clique would use biological agents if the opportunity arose. One threat that loomed in the aftermath of September 11 was the possibility that bin Laden's followers would reintroduce the smallpox virus, which had been eradicated by worldwide vaccinations during the 1970s. The reason why smallpox could pose such a great threat is precisely that it has been eliminated, and few Americans born after 1973 have received vaccines. Unless they gained access to one of the two labs worldwide (one in the United States and one in Russia) where smallpox virus is stored for the purpose of making vaccines, however, terrorists would be unable to obtain a sample. (It is this matter of access that led authorities to suspect that the anthrax attacks were an "inside job.")

Another biological agent that poses a threat is Clostridium botulinum, which causes botulism, a toxic condition that can result in paralysis. Members of the fanatic Japanese cult Aum Shinrikyo attempted unsuccessfully to launch botulism attacks in Tokyo on three occasions in 1995. The Japanese government itself—that is, the Axis Japanese government of World War II—experimented with another biological agent, tularemia, or Francisella tularensis. The pathogen, which causes lung inflammation and death, is considered one of the most dangerous forms of biological weapon, because it is extremely efficient and easy to spread. America's military, borrowing an idea from its former enemy, developed its own F. tularensis strain in the late 1960s but destroyed its stockpile in 1973.

Where to Learn More

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

Cranmer, Hilarie. Anthrax Infection. Emedicine.com (Web site). <http://www.emedicine.com/emerg/topic864.htm>.

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

Everything You Need to Know About Diseases. Spring-house, PA: Springhouse Corporation, 1996.

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

Hoff, Brent H., Carter Smith, and Charles H. Calisher. Mapping Epidemics: A Historical Atlas of Disease. New York: Franklin Watts, 2000.

Infection and Immunity. University of Leicester Microbiology and Immunology (Web site). <http://wwwmicro.msb.le.ac.uk/MBChB/MBChB.html>.

Marr, Lisa. Sexually Transmitted Diseases: A Physician Tells You What You Need to Know. Baltimore, MD: Johns Hopkins University Press, 1998.

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

Shein, Lori. AIDS. San Diego: Lucent Books, 1998.

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A pathological condition spread among biological species. Infectious diseases, although varied in their effects, are always associated with viruses, bacteria, fungi, protozoa, multicellular parasites and aberrant proteins known as prions. A complex series of steps, mediated by factors contributed by both the infectious agent and the host, is required for microorganisms or prions to establish an infection or disease. Worldwide, infectious diseases are the third leading cause of human death.

The most common relationship between a host and a microorganism is a commensal one, in whichadvantages exist for both organisms. For example, hundreds of billions of bacteria of many genera live in the human gastrointestinal tract, coexisting in ecological balance without causing disease. These bacteria help prevent the invasion of the host by more virulent organisms. In exchange, the host provides an environment in which harmless bacteria can readily receive nutrients. There are very few microorganisms that cause disease every time they encounter a host. Instead, many factors of both host and microbial origin are involved in infectious disease. These factors include the general health of the host, previous exposure of the host to the microorganism, and the complement of molecules produced by the bacteria.

Spread of a pathogenic microorganism among individual hosts is the hallmark of an infectious disease. This process, known as transmission, may occur through four major pathways: contact with the microorganism, airborne inhalation, through a common vehicle such as blood, or by vector-borne spread.

The manner in which an infectious disease develops, or its pathogenesis, usually follows a consistent pattern. To initiate an infection, there must be a physical encounter as which the microorganism enters the host. The most frequent portals of entry are the respiratory, gastrointestinal, and genitourinary tracts as well as breaks in the skin. Surface components on the invading organism determine its ability to adhere and establish a primary site of infection. The cellular specificity of adherence of microorganisms often limits the range of susceptible hosts. For example, although measles and distemper viruses are closely related, dogs do not get measles and humans do not get distemper. From the initial site of infection, microorganisms may directly invade further into tissues or travel through the blood or lymphatic system to other organs.

Microorganisms produce toxins that can cause tissue destruction at the site of infection, can damage cells throughout the host, or can interfere with the normal metabolism of the host. The damage that microorganisms cause is directly related to the toxins they produce. Toxins are varied in their mechanism of action and host targets. See also Cholera; Staphylococcus.

The host's reaction to an infecting organism is the inflammatory response, the body's most important internal defense mechanism. Although the inflammatory response is also seen as secondary to physical injury and nonspecific immune reactions, it is a reliable indicator of the presence of pathogenic microorganisms. Immune cells known as lymphocytes and granulocytes are carried by the blood to the site of infection. These cells either engulf and kill, or secrete substances which inhibit and neutralize, microorganisms. Other white blood cells, primarily monocytes, recognize foreign organisms and transmit chemical signals to other cells of the host's immune system, triggering the production of specific antibodies or specialized killer cells, both of which are lethal to the infecting microorganism. Any influence that reduces the immune system's ability to respond to foreign invasion, such as radiation therapy, chemotherapy, or destruction of immune cells by an immunodeficiency virus such as HIV, increases the likelihood that a organism will cause disease within the host. See also Inflammation.

Chemical compounds that are more toxic to microorganisms than to the host are commonly employed in the prevention and treatment of infectious disease; however, the emergence of drug-resistant organisms has led to increases in the morbidity and mortality associated with some infections. Other methods for controlling the spread of infectious diseases are accomplished by breaking a link in the chain of transmission between the host, microorganism, and mode of spread by altering the defensive capability of the host. Overall, the three most important advances to extend human life are clean water, vaccination, and antibiotics (in that order of importance).

Water-borne infections are controlled by filtration and chlorination of municipal water supplies. Checking food handlers for disease, refrigeration, proper cooking, and eliminating rodent and insect infestation have markedly reduced the level of food poisonings. The transmission of vector-borne diseases can be controlled by eradication of the vector. Blood-borne infections are reduced by screening donated blood for antibodies specific for HIV and other viruses and by rejecting donations from high-risk donors. For diseases such as tuberculosis, the airborne spread of the causative agent, Mycobacterium tuberculosis, can be reduced by quarantining infected individuals. The spread of sexually transmitted diseases, including AIDS, syphilis, and herpes simplex, can be prevented by inhibiting direct contact between the pathogenic microorganism and uninfected hosts. See also Acquired immune deficiency syndrome (AIDS); Food poisoning; Vaccination; Water-borne disease.

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Infectious diseases are the result of damaging microorganisms obtaining access to the body, and not being repelled or destroyed by the immune system. Their relationship to man is that of parasite and host, and is continually adjusting. Numerous different types of bacteria, viruses, and other organisms may cause disease, and infection may take place through close contact with an infected person, or through the respiratory, digestive, or genito-urinary systems, depending on the organism and the disease involved. Infection may also occur by transmission from an animal, or via an insect vector. Organisms can damage the body by their multiplication in or around its cells, or by the widespread poisonous effects of substances (toxins) which they release. Many infectious diseases are of a self-limiting character, ending in either full recovery or death. While certain of them may occasionally have long-term sequelae, the body, if its defences win, for the most part returns to normal, often retaining a lifelong subsequent immunity against that specific infection. Other infections, such as syphilis, are, however, chronic, and eventually fatal if untreated.

Diseases included in the category ‘infectious’ include colds and influenza; the familiar infectious illnesses of childhood, and the more serious conditions such as poliomyelitis, diphtheria and meningitis, typhoid, typhus, cholera, dysentery, and smallpox. Tuberculosis is also an infectious disease, although its clinical progress is chronic rather than acute.

Most of these diseases have a very ancient history. While many only emerge as identifiable entities in the medical writings of the seventeenth and eighteenth centuries, others can be demonstrated to have been present in antiquity. Smallpox, for example, which was declared eradicated by the World Health Organisation in 1977, can clearly be identified by characteristic lesions on the mummified corpses of ancient Egyptians, while a stele of the same civilization, dating from 1580-1350 bc, shows a young man displaying a withered and shortened left leg, held in the ‘equinus position’ characteristic of paralysis possibly caused by poliomyelitis. Infectious diseases also occur in the animal kingdom, and some, such as anthrax and yellow fever, are transmissible to man.

Understanding

While the closely allied concepts of infection and contagion (transmission of disease from one person to another by direct or indirect contact) are probably almost as old as mankind, it was only in the mid nineteenth century, with the development of accurate microscopes and of laboratory research, that these processes began to be scientifically elucidated. Several observers indicated the likelihood of microorganisms as causal agents of disease, and even detected their paths of transmission, such as the faecal-oral route for typhoid and cholera, but it was Louis Pasteur who, in the early 1860s, first gave a coherent account of the process of infection in what is popularly known as the germ theory of disease. In 1876, Robert Koch identified the causal organism of anthrax, and within a few years had also identified the agents of tuberculosis and cholera. By 1900, the specific agents of numerous diseases had been identified, and the diverse routes of transmission — of infection and contagion — were beginning to be mapped out.

Infectious diseases are often ‘crowd diseases’, which depend for the most part on reservoirs of susceptible people to maintain themselves. Person to person infections, for example, are thought to have become more apparent between 3000 and 500 bc, when urban centres grew large enough to support them. These diseases soon established an endemic character in such centres, meaning that the diseases or infectious agents were constantly present in that area. City populations, exposed early in life, acquired high levels of immunity to them, compared with rural populations. Rapid and unregulated urban growth brought a great escalation in the incidence of and mortality from many of these diseases in Western Europe and North America during the nineteenth century, and several, including tuberculosis, typhoid, measles, and whooping cough, were responsible for much human misery and many thousands of deaths. Recurrent gastro-intestinal infections, in particular, helped to undermine the health, and natural resistance to infection, of babies and young children, and indeed of adults also. By 1830, annual death rates of over 30 per 1000 living persons were commonplace in Western cities, while infant mortality rates rivalled those of under-developed nations today.

Prevention

Beginning in the 1830s, public health movements began to develop in many Western states in response to this crisis of mortality. For example, in Britain — one of the first nations to begin to adopt public health measures — early reformers such as Edwin Chadwick stressed the enormous economic costs of such a wastage of life. At this period, notions of contagion marched in parallel with a belief that gases generated by rotting organic matter were productive of epidemics, and early attempts at preventing premature deaths focused on environmental improvement. Slowly and painfully, through the following decades, filtered and piped water systems, mains drainage, systematic scavenging, and slum clearance brought about cleaner, healthier urban environments, and disrupted the transmission routes of a number of important infections, notably of water-borne typhoid and cholera and of louse-borne typhus.

The development of specific methods of prevention came late in the history of the infectious diseases. Smallpox, one of the most ancient and most hideous diseases, was the first to be tackled in this way. At some point, the Chinese had discovered that by introducing matter taken from smallpox vesicles into a scratch on the normal healthy body, controlled, immunizing infections could be established. This method, the inoculation of material containing the living organism, itself was not foolproof, since it was not possible to ensure a mild rather than a virulent infection, which might prove fatal. Nonetheless, knowledge of the technique spread along trade routes to Turkey, and thence to Europe in the early eighteenth century. In 1796, a Gloucestershire medical practitioner, Edward Jenner, picked up on local lore which suggested that infections with cow-pox would protect against smallpox, and demonstrated that this was indeed the case. This practice, vaccination (from vaccinus: pertaining to a cow) was later refined, and, encouraged by many European governments, the introduction of the modified or related organism displaced inoculation as the principal preventive against smallpox. At this stage, however, the processes and principles which made vaccination effective were still not understood.

Smallpox vaccination represented an ideal for disease eradication which provided an important model for future medical research. Louis Pasteur, for example, set out in his later career to investigate the principles of immunology with a view to understanding how vaccination worked. Pasteur's breakthrough with the principle of attenuating viruses — reducing their virulence — came in 1876. This meant that the body's immunity to subsequent infection by a virulent organism could be actively provoked in response to a non-threatening form of the same strain; Pasteur proceeded to develop immunizations against various animal diseases, including anthrax and rabies. It was his reluctant application of rabies vaccine to the boy Joseph Meister in 1883 that first alerted the general public to the eventual possibilities of immunology.

As the discipline developed through the work of Pasteur, his colleagues, and his successors, new therapeutic and preventive indications emerged. Early successes came for diphtheria in 1894 with anti-toxin therapy (the use of material produced by the inoculation of animals with toxins produced by bacteria), and for both diphtheria and tetanus with the development of active immunization (the production of protective antibodies by stimulating the body's immune system). In 1896, Almroth Wright succeeded in producing an anti-typhoid vaccine using killed bacteria, thus extending the theoretical options for vaccine development. In the interwar period, successful vaccines were developed against diphtheria and tuberculosis, and, in the years following World War II, they were developed against most of the principal childhood infections — whooping cough, poliomyelitis, German measles, and measles, and eventually against mumps and chicken-pox as well.

Since 1870, there has been an enormous decline in death rates from infectious diseases in developed countries. This decline has been hastened by the availability of immunizations, but in most cases had begun well before such protection was available. Rising living standards — including smaller families, better housing, improved domestic hygiene, a reduced incidence of gastro-intestinal infections, and better nutrition — together with public health measures contributed largely to this reduction. Many childhood diseases remain serious in poor and under-developed countries. Immunization, although a valuable resource with some diseases, is by no means a viable prospect for all infections; despite decades of research, no vaccine has yet been approved for malaria, one of the world's most serious infections.

New infections

New infectious diseases are still emerging, and there is no room for complacency in this regard. The emergence of poliomyelitis as a serious killer and maimer between about 1911 and 1962 was partly attributable to improved hygienic standards in the West, which meant that children were no longer harmlessly exposed to the virus as babies. Lassa fever, exemplar of a whole new generation of sinister tropical fevers, emerged in Nigeria in 1969, while Legionnaires' disease was identified in the US in the 1970s. The rapid global spread of HIV infection since 1980 echoes that of syphilis in Europe in the fifteenth century. Epidemics of the terrifying Ébola virus in Zaire, and of bubonic plague in India in the early 1990s, indicate that both new and old infections retain the potential for major human tragedy. One consequence of global warming could possibly be the reappearance of malaria as an indigenous infection in parts of the world which have been free of it for many decades. Relentless human exploitation of tropical resources, uncontrolled human reproduction, increased travel, and unregulated technological development all create the potential for unleashing fresh manifestations of new and old infections by disturbing global environmental equilibrium.

— Anne Hardy

Bibliography

• Garrett, L. (1996). The coming plague: newly emerging diseases in a world out of balance. Penguin Books, London.
• McNeill, W. H. (1979). Plagues and peoples. Penguin Books, Harmondsworth

See also antibiotics; epidemic; fever; immune system; immunization; microorganisms; sexually transmitted diseases.

Diseases that are caused by micro-organisms such as bacteria and viruses, many of which are spread from person to person. An intermittent host, or vector, aids the spread of some infectious diseases.

Diseases caused by the growth of pathogenic organisms in the body. Some of these diseases may also be contagious diseases.

Diseases of man or animal that are transferred by the ingestion of food substances, insect bites, or other causes. Usually the introduction of the organism into the body is by way of direct introduction into the bloodstream or into the bloodstream via intestinal absorption. See Diseases, Pathogenic (Pathological) Organisms.

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Pathologic alterations induced in the tissues by the action of microorganisms and/or their toxins. Some infectious diseases involving the oral tissues are herpes zoster, herpetic gingivostomatitis, moniliasis, syphilis, and tuberculosis.

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