First report of AIDS (Pneumocystis pneumonia cluster)

Pneumocystis carinii pneumonia was, in 1981, a disease of the immunocompromised: organ transplant recipients on azathioprine, patients on high-dose corticosteroids, children with congenital immune deficiencies. Between October 1980 and May 1981, five previously healthy young men, all gay, were treated for biopsy-confirmed Pneumocystis pneumonia at three Los Angeles hospitals. Michael Gottlieb, an immunologist at UCLA, and colleagues at UCLA and Cedars-Mt. Sinai Hospital sent the cases to the CDC's weekly bulletin.

The CDC's MMWR published the report on June 5, 1981. The men were 29 to 36 years old. All five also had cytomegalovirus infection and candidal infection of the mucosa, and the three who were tested had severely depressed lymphocyte responses. Two had died before the report appeared. The editors noted that in the United States Pneumocystis pneumonia was almost exclusively limited to severely immunosuppressed patients. An editorial note linked the cases to some aspect of a homosexual lifestyle or a sexually acquired disease and raised the possibility of a cellular immune defect related to a common exposure.

A second report, on July 3, 1981, described Kaposi's sarcoma, usually a rare and indolent tumor, in 26 gay men in New York and California. Together, the two clusters pointed to a transmissible agent capable of destroying cellular immunity. James Curran at the CDC organized the task force that began systematic epidemiological surveillance. Cases then appeared in intravenous drug users, hemophiliacs receiving clotting factor concentrates, Haitian immigrants and infants given blood transfusions, so the disease was clearly not confined to sexual contact within one group. The CDC first used the term AIDS, for acquired immune deficiency syndrome, on September 24, 1982.

The causative virus was identified in 1983. Luc Montagnier and Francoise Barre-Sinoussi at the Pasteur Institute in Paris isolated a retrovirus from lymph node tissue of a patient with lymphadenopathy and reported it in Science on May 20, 1983. On April 23, 1984, the US health secretary announced that Robert Gallo's group at the National Cancer Institute had found the cause, a retrovirus they called HTLV-III; their papers in Science that year reported a way to grow it in culture, which made antibody testing possible. Montagnier and Barre-Sinoussi received the Nobel Prize in 2008.

The virus, eventually named HIV, spread from the initial clusters into a global epidemic. The five Los Angeles cases were followed by millions more worldwide. WHO estimates that HIV has claimed about 44.2 million lives, and that 41.0 million people were living with HIV at the end of 2025.