The first cases of what has become known as the acquired immunodeficiency syndrome (AIDS) were reported in mid-1981 from Los Angeles, California. One month following these five reports of Pneumocystis carinii pneumonia (PCP) in young homosexual men, 26 cases of Kaposi’s sarcoma (KS) in homosexual men in New York and California and additional cases of PCP and other opportunistic infections were reported. Reports of cases in the United States continued to rise, and soon the occurrence of PCP, KS, or other serious opportunistic infections in a person with unexplained immune dysfunction became known as AIDS. In retrospect, sporadic cases may have occurred in the United States, Europe, or Africa as much as three decades earlier, but the worldwide epidemic was not apparent until the 1980s.

The initial occurrence of AIDS in homosexual men and injecting drug users (IDUs) suggested by 1982 that a transmissible agent was the likely cause. The transmissible agent hypothesis gained credence by early 1983 with the documented occurrence of AIDS in persons with hemophilia and in recipients of blood transfusions.

Within a year, the retrovirus, now termed human immunodeficiency virus (HIV), was isolated and shown to be the cause of AIDS.
Recent data support the theory that the HIV virus originated in Africa. Blood obtained in 1959 from an adult Bantu man in the Democratic Republic of Congo represents the oldest known HIV-1 virus infection.