A progressive, degenerative disease involving several major organ systems, including the immune system and central nervous system. Uniformly fatal, it is associated with human immunodeficiency virus (HIV), a viral infection that progressively weakens the immune system.

Since Acquired Immune Deficiency Syndrome (AIDS) manifests itself in a number of different diseases and conditions, it has been difficult to arrive at a formal definition. In an attempt to standardize the definition of AIDS, the Centers for Disease Control in 1992 included among its diagnostic criteria a count of 200 or fewer CD4T lymphocyte cells per cubic ml of blood (a sign of severe immune system suppression). AIDS was first recognized in 1981 as a cluster of symptoms in homosexual men in New York City and San Francisco. Eventually, similar symptoms were found among intravenous drug users, hemophiliacs, and other recipients of blood...