HIV-1 in humans resulted from at least four cross-species transmissions of simian immunodeficiency viruses (SIVs) from chimpanzees and gorillas in West Central Africa, while HIV-2 viruses resulted from at least eight independent transmissions of SIVs infecting sooty mangabeys in West Africa only, where one of these transmissions (HIV-1 group M) is responsible for the global epidemic. HIV-1 M is subdivided into nine subtypes and a wide diversity of circulating recombinant forms (CRFs) and unique recombinant forms. The heterogenic HIV-1 M subtype/CRF distribution is the result of founder effects. The genetic diversity of HIV-1 continues to increase overtime due to demographic factors such as travel and migration and frequent co/superinfections. In addition, the expanded access to antiretrovirals leads to an increasing number of drug-resistant strains, especially in resource limited countries.