There is persuasive evidence that HIVs are derived from multiple cross-species transmissions of simian immunodeficiency viruses (SIVs) that naturally infect African non-human primates. The pandemic HIV-1 strain of group M originated from a single transmission event from a chimpanzee that harbours SIVcpz near Cameroon in Central Africa. Multiple other transmission events of SIVs from chimpanzees to humans were also detected, but their resulting HIV-1 viruses in groups N, O and P spread in humans only to a limited extent [44]. Group O was found in a few tens of thousands of people in West-Central Africa. Groups N and P were identified in 13 and 2 individuals, respectively. Likewise, appreciable spreading of HIV-2 within humans is seen only with groups A and B resulting from two cross-species transmissions of SIVsmm from sooty mangabeys in West Africa [44]. All other groups (C–H) were found only in single individuals. Thus, both HIV-1 and HIV-2 originated from one or two primate-to-human transmission events. The other transmission events were unproductive, representing incidents in which secondary and tertiary spreading was very limited.