The vast majority of researchers now accept that AIDS is caused by a virus that was identified in France in 1983 and the United States in 1984. This was called either lymphadenopathy-associated virus (LAV) or human T-cell lymphotropic virus type III (HTLV-III). Since 1986, it has generally been referred to as HIV.

In 1984, a test for detecting antibodies to HIV was developed and this revealed that only a relatively small proportion of people with HIV had gone on to develop AIDS: AIDS was simply one end of a spectrum of different effects of HIV infection, ranging from staying well through to life-threatening opportunistic illnesses.

In 1986, HIV-2 was discovered, and some researchers are claiming there may be other different strains of the virus. Fortunately, these all appear to behave in much the same way, so that this need not change our advice about transmission, prevention, safer sex and hygiene. HIV-2 has not been studied as much as HIV-1, but it causes less severe immune damage than HIV-1 in the long term.