The skin is subject to localized and generalized disorders, as well as those of primary occurrence in the skin and those secondary to involvement of other tissue. Diseases and disorders may affect any of the structures of the skin. They may be caused by external agents, either infectious or noninfectious, or by the abnormal accumulation of normal or abnormal skin elements, either inborn or acquired.
Infectious diseases
These are classified by the type of infectious agent—bacterial, parasitic, fungal, or viral.
Bacterial infections are distinguished clinically by the skin layer or appendage affected, but treatment is based upon the organism causing the infection. Impetigo and cellulitis are the most common infections of skin. Both infections may be caused by streptococcus, including group B hemolytic streptococcus, and staphylococcus, often species resistant to penicillin. Skin appendages such as hair follicles may be similarly infected. See also Antibiotic; Drug resistance; Staphylococcus; Streptococcus.
Parasitic skin conditions are most often seen in epidemics among individuals who are in close contact, or where hygiene is poor. Head, body, and pubic lice, and scabies are the most common. Pubic lice and scabies are also often transmitted by sexual contact. See also Medical parasitology.
Fungal infections are extremely common. In the United States, most fungal infections in humans are with species incapable of infecting tissue other than keratinized epidermis. These organisms are known as dermatophytes; they cause tinea pedis (athlete's foot), tinea cruris (some forms of jock itch), and tinea capitis (a scalp condition responsible for some forms of hair loss). See also Medical mycology.
Viral diseases often involve the skin; however, viral warts (verruca vulgaris) and molluscum contagiosum are the primary examples of viral diseases that affect only the skin. Both are characterized by single or multiple, somewhat contagious, skin tumors that usually are small but can in rare instances exceed 0.4 in. (1 cm) in diameter. See also Animal virus; Chickenpox and shingles; Herpes; Measles; Smallpox.
Inflammatory disease
Most itchy rashes are due to inflammation of the skin; they are usually known as eczema or dermatitis. In the acute stage, eczematous dermatitis is characterized by a vesicular, oozing condition. Seborrheic dermatitis is a common eczematous condition affecting primarily the areas of skin that bear sebaceous glands, that is, scalp, central face, chest, axilla, and groin.
Hereditary disease
Atopic dermatitis is the skin manifestation of atopy, a clinically apparent hypersensitivity. This condition may also be associated with asthma and pollen allergies. Psoriasis is a common disease of unknown etiology. The typical psoriasis consists of well-defined patches and plaques of red skin with a silvery scale that often results in pinpoint bleeding when removed. The most common and persistent sites are the elbows, knees, and scalp, but any area of skin may be involved; there may be significant morbidity and disability.
Other conditions
Acne vulgaris is an extremely common skin disorder, affecting 80–90% of young adults, usually during adolescence. The pilosebaceous organ of the skin is the primary target, particularly the sebaceous gland, its duct, and the infundibulum of the hair follicle. In addition to increased production of sebum, bacteria contribute to the development of acne lesions. These lesions consist of open and closed comedones (blackheads), papules, pustules, and cysts.
Reactions to ingested materials, such as food or medications, often appear in the skin, usually either as a red, itchy, measleslike rash or as urticaria (hives). These reactions indicate allergy to the material, which must then be avoided. See also Allergy.
Neoplasms
Skin neoplasms may be benign or malignant, congenital or acquired, and they may arise from any component of the skin. Almost all skin neoplasms are benign and acquired. The common mole (melanocytic nevus) is a neoplasm of benign melanocytes; usually it is acquired, but it may be present at birth, when it is often known as a birthmark. Other common congenital nevi or birthmarks are of vascular origin, including strawberry and cavernous hemangiomas and port wine stains.
Malignant neoplasms may arise from cellular elements of the epidermis or dermis, or by infiltration of the skin by malignant cells arising from other tissues. By far the most common are basal cell and squamous cell cancers, which arise from basal and squamous keratinocytes of the epidermis, respectively. They usually are characterized by a nonhealing sore, persistent red scaling or crusting patch, or a slowly growing pearly nodule on skin that has been exposed to the sun; they occur mostly on the head, neck, hands, and arms.
Malignant melanoma arises from the pigment-forming melanocyte, and thus it is usually pigmented. It is a metastasizing cancer that is often fatal if not removed surgically in the early stage. It can be recognized as a pigmented lesion, often thought to be a benign nevus initially, that increases in size, changes color, particularly with admixtures of black, blue, red, or white along with the usual tans or browns; and it becomes irregular in size and shape. See also Cancer (medicine).
Two unusual multicentric primary skin malignancies are mycosis fungoides and Kaposi's sarcoma. Mycosis fungoides is a lymphoma of the skin, that may remain confined to the skin for 10 or more years before eventually spreading to internal organs and causing death. It may be extremely difficult to diagnose, both clinically, when it can appear only as eczematous patches, and histologically, for months or years. Kaposi's sarcoma occurs in two forms, the classic form seen on the legs of elderly Mediterranean men, and a form associated with HIV-1 infection and AIDS that may occur on any part of the body. It is derived from skin blood vessels, is multicentric, and usually appears as red to violet patches, plaques, or nodules. It is usually not fatal, although it may eventually spread to internal organs and may cause significant morbidity. There are numerous other primary skin cancers, but they are rare. See also Acquired immune deficiency syndrome (AIDS); Skin.
