The ecologies of disease are examined to elucidate how biological, social, and cultural factors may contribute to both the occurrence and the prevention of disease. The human immune system and how it functions to control disease and infection are discussed. The ecological niche of the AIDS virus is considered within the context of factors that change the event of transmission, ranging from the cost of air travel to the use of condoms. The AIDS viruses are described, and the method by which the virus invades the immune system and affects its functioning is delineated. The progression of human immunodeficiency virus infection from initial symptoms through full-blown cases of AIDS is outlined. Symptoms at various stages of infection, prognosis, and treatment also are described. The AIDS antibody test, testing policies, and the costs and benefits of testing are considered, with particular emphasis on discrimination, confidentiality, and prevention issues. The risks associated with the AIDS epidemic and the personal choices necessitated by them are examined, with focus on the need for modifying sexual and drug use behaviors. The ways in which the AIDS epidemic mirrors social and cultural dilemmas are outlined, and implications of the epidemic for society are discussed. The need for changes in attitudes, behaviors, values, and policies in dealing with the threat of AIDS is emphasized. Index and suggested readings.