The current report focuses on HIV-2, one of the two major types of the virus. The other major type is HIV-1.

Both cause AIDS and are transmitted through sex or infected blood and from mother to child. But HIV-2 is less readily transmitted and appears to progress to AIDS more slowly than HIV-1.

Although infection with HIV-1 has progressed into a worldwide epidemic, HIV-2 infections have remained largely confined to West Africa, where the virus has infected approximately one percent of the population.

HIV closely resembles a virus that infects primates, known as SIV. However, HIV-1 appears most closely related to the strain of SIV that is present in chimpanzees, while HIV-2 appears to have its predecessor in a strain of SIV present in sooty mangabeys.

By analyzing HIV-2 samples taken from people, and comparing them to SIV samples taken from sooty mangabeys and other primates that acquired SIV from sooty mangabeys, the researchers estimate that the two subtypes of HIV-2 that became epidemics first infected humans around 1940 and 1945, according to a report in the Proceedings of the National Academy of Sciences.

Study author Dr. Anne-Mieke Vandamme of Katholieke Universiteit Leuven in Belgium told Reuters Health that HIV-2 may have crossed from sooty mangabeys to humans as a result of bushmeat slaughtering or hunting, the same process that may have enabled HIV-1 to infect humans.

The researchers also discovered evidence suggesting that the West African country Guinea-Bissau, the presumed site of origin of HIV-2, experienced a significant increase in new HIV-2 infections between 1955 and 1970.

And that epidemic continues today, Vandamme noted.

"Since 1970, it is still epidemic up to the time of sampling of the sequences used in the analysis, which is 1991," she said.

This African region experienced a war of independence against the Portuguese between 1963 and 1974. The fact that the dramatic spread of HIV-2 in the region coincided with this event suggests that war may have encouraged an increase in infections in the region, according to Vandamme and colleagues.

War may have spawned an HIV-2 epidemic in this region by increasing the number of people who received unsterile injections in hospitals, the authors suggest. Moreover, reports from the region note that army-trained doctors started campaigns to inoculate residents of Guinea-Bissau.

Indeed, the first reported cases of HIV-2 in Europe occurred in Portuguese soldiers returned from the independence war, the authors note.

SOURCE: Proceedings of the National Academy of Sciences 2003;10.1073/pnas.0936469100.