The human AIDS viruses – HIV-1 and HIV-2 – impose major burdens on the health and economic status of many developing countries. Surveys of other animal species have revealed that related viruses – the SIVs – are widespread in a large number of African simian primates where they do not appear to cause disease. Phylogenetic analyses indicate that these SIVs are the reservoirs for the human viruses, with SIVsm from the sooty mangabey monkey the most likely source of HIV-2, and SIVcpz from the common chimpanzee the progenitor population for HIV-1. Although it is clear that AIDS has a zoonotic origin, it is less certain when HIV-1 and HIV-2 first entered human populations and whether cross-species viral transmission is common among primates. Within infected individuals the process of HIV evolution takes the form of an arms race, with the virus continually fixing mutations by natural selection which allow it to escape from host immune responses. The arms race is less intense in SIV-infected monkeys, where a weaker immune response generates less selective pressure on the virus. Such a difference in virus-host interaction, along with a broadening of co-receptor usage such that HIV strains are able to infect cells with both CCR5 and CXCR4 chemokine receptors, may explain the increased virulence of HIV in humans compared to SIV in other primates.