The acquired immunodeficiency syndrome (AIDS) is caused by two retroviruses of the lentivirus subfamily, human immunodeficiency viruses types 1 (HIV-1) and 2 (HIV-2), which derive from cross-species transmissions of viruses naturally infecting two African primate species, the chimpanzee (Pan troglodytes) and the sooty mangabey (Cercocebus atys), respectively. Eleven transmission events of lentiviruses from these primate species to humans have been inferred from phylogenetic sequence analyses, although only four (which originated HIV-1 groups M and O, and HIV-2 groups A and B) have resulted in epidemic spread. One of these transmissions, corresponding to the common ancestor of the HIV-1 group M viruses, whose initial expansion in Central Africa has been dated by molecular clock analyses to around 1930, eventually originated the AIDS pandemic. How humans acquired the ancestors of the AIDS viruses from simians is uncertain, but natural infection of African non-human primates with lentiviruses is common, and contact with blood of these animals, which are frequently hunted in Africa for human consumption, is a plausible mechanism for virus acquisition. Sociodemograhic developments, including rapid urbanization, large scale migrations, growth of prostitution, wars, and increase in travel, and, possibly, the widespread use of nonsterile injection material, may have promoted the emergence of the AIDS pandemic in Central Africa in the 20th century. Subsequent global propagation of HIV-1 derives from multiple introductions at different time points, often representing phylogenetically identifiable founder events. In areas in which diverse HIV-1 variants are cocirculating, these frequently recombine to generate mosaic forms, some of provided new insights on the origin and spread of AIDS viruses and may contribute to the efforts aimed at containing the pandemic which have propagated extensively. Phylogenetic sequence analyses and molecular epidemiology studies have.