The evolution of HIV-1 and the origin of AIDS
"At the time when HIV-1 was first described, the closest known relative was visna, a virus from sheep that is the prototypic member of the genus Lentivirus. Additional lentiviruses were soon found in other primates, and a second virus (HIV-2) was found infecting humans.
The viruses from non-human primates were termed simian immunodeficiency viruses (SIVs). Among the first species to be found to be naturally infected were African green monkeys (Chlorocebus species), where the prevalence of infection is high (greater than 50% of adults) and natural infections appear to be non-pathogenic.
The number of different SIVs identified has increased steadily over the past 20 years. Currently, around 40 different primate species have been found to harbour SIVs, though information regarding prevalence and pathogenicity is lacking for most. So far, SIVs have only been found naturally infecting primates in sub-Saharan Africa, though the extent to which Asian or new world primates have been surveyed is unclear. Where multiple strains of SIVs have been characterized from a single species, they generally form a monophyletic clade, indicating that the great majority of transmissions are intraspecific. The primate viruses as a whole, including HIV-1 and HIV-2, form a distinct clade within the lentiviruses, indicating that humans acquired their infections from other primates (Bailes et al. 2002). Phylogenetic analyses of these primate lentiviruses have provided remarkably detailed insights into the evolutionary origins of the human viruses."
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