In 1981, doctors in the United States began to notice a series of unusual infections in gay men in Los Angeles, San Francisco, New York and other big cities. These infections had previously been extremely rare, except amongst people whose immune systems had been seriously weakened in some way. The most life-threatening seemed to be Pneumocystis pneumonia (PCP) and Kaposi's sarcoma, but other severe infections and tumours were also detected amongst gay men in these cities.

These problems were first reported in the 5 June 1981 edition of Morbidity and Mortality Weekly Report, the Centers for Disease Control and Prevention’s (CDC’s) bulletin for doctors. The report, from doctors in Los Angeles, came about because of a sharp increase in requests for a drug used to treat PCP, which was only available through the CDC at that time. Staff spotted a pattern and the report subsequently brought the syndrome to the attention of doctors in other cities in the United States.

By late 1981, researchers were beginning to link these `opportunistic infections' to immune system damage. Many of the patients had significantly altered ratios of CD4 T-cells (also known as `T-helper cells') to CD8 T-cells (`cytotoxic cells'). This balance is an essential component of one part of the immune system that helps the healthy body fight off infection.