Simian immunodeficiency virus

retrovirus found in primates; the humans species is known as HIV.

Since the isolation of an HIV-2-related virus from captive macaques (SIV-mac), the origin of human immunodeficiency viruses, a much debated subject, has been attributed to monkeys. The sequence of SIV-agm, which is derived from a naturally infected African green monkey, shows equal relatedness to HIV-1 and HIV-2, suggesting that the derivation of these viruses from SIV-agm is unlikely. Recent sequence analysis of SIV from a captive sooty mangabey (SIV-sm), however, shows its close relatedness to HIV-2 and SIV-mac, indicating a possible origin of HIV-2 and SIV-mac from SIV-sm. The sequence of a novel simian lentivirus, SIV-mnd, isolated from a wild-caught mandrill in Africa is distinct from the three other main groups, HIV-1, HIV-2/SIV-mac/SIV-sm and SIV-agm, and therefore represents a fourth main group of primate lentiviruses. Phylogenetic analysis indicates that these four main virus groups might have diverged from a common ancestor at about the same time, long before the spread of AIDS in humans.