AIDS is an apparently new condition that first occurred in about 1979 and is manifested primarily by profound disturbances of T-cell immunity and unusual susceptibility to either opportunistic infections (mycobacterial, fungal, parasitic, or viral) or tumors such as Kaposi's sarcoma and lymphoma. A prodrome of lymphadenopathy and wasting is also part of the spectrum of disease. The etiology is unknown, but the likely candidates are viral agents. Epidemiologic studies show that the promiscuous gay male is at particular risk, but transmission by the parenteral route and/or vertical transmission has extended the risk of acquisition to other groups. A common clinical presentation of full-blown AIDS is opportunistic infection of the lungs (especially with P. carinii), the central nervous system, or the gastrointestinal tract. Therapy against specific pathogens may be temporarily successful, but the course is frequently downhill, with relentless progressive or sequential infections or development of a tumor during continued immunodepression. Therapy for the immunologic abnormalities remains experimental. Control measures to prevent transmission are similar to those for hepatitis B.