The two known types of HIV are members of a family of primate lentiviruses. HIV, like other retroviruses, contains a virus capsid, which consists of the major capsid protein, the nucleocapsid protein, the diploid single-stranded RNA genome, and the viral enzymes protease, reverse transcriptase, and integrase. HIV isolates show extensive genetic variability, resulting from the relatively low fidelity of reverse transcriptase in conjunction with the extremely high turnover of virions in vivo. These features of HIVs may have strong implications for vaccine development. Simian immunodeficiency viruses from naturally infected animals differ from HIV in one fundamental respect: they do not cause disease in their natural hosts. Study of these viruses may therefore lead to information about the interaction between lentiviruses and host immune response that could be exploited to combat AIDS.