Previous research had identified two genetically distinct viruses, dubbed HIV-1 and HIV-2, that cause AIDS in humans. Both types were transmitted from simians (HIV-1 from chimpanzees and HIV-2 from sooty mangabeys) but they currently have very different distribution patterns among people. Whereas subtypes of HIV-1 are spread globally, those of HIV-2 are largely restricted to West Africa. Philippe Lemey of Katholieke Universiteit Leuven in Belgium and his colleagues analyzed historic and modern samples of HIV-2 from a rural population in Guinea-Bissau, as well as a SIV-SM strain from sooty mangabeys. (In the image above, a girl from Sierra Leone carries a pet sooty mangabey.) By comparing the different genomes, the team estimated the number of mutations in the sequences that had occurred over time and determined that the virus jumped from primates to humans twice, once around 1940 and again in 1945.
The researchers also ascertained that the incidence of infection with the subtype of HIV-2 that was first transmitted to humans in 1940 reached epidemic proportions between 1955 and 1970. This timing coincides with Guinea-Bissau's fight for independence from Portugal, which occurred between 1963 and 1974. In particular, they note that army-trained doctors launched a massive inoculation program in the region, and suggest that the difficulties of running such a campaign during war-time could have led to an increase in unsanitary injections that may have spread of the disease. Diagnosis of HIV-2 cases among Portugese war veterans also supports a hypothesis of increased sexual transmission of the disease during the war. The authors thus conclude that their work "strongly supports the major role of the independence war and associated changes in sociocultural patterns in the transition of HIV-2 from endemic to epidemic behavior."