"It says that the chimpanzee group that gave rise to HIV ... this chimp community resides in Cameroon," said Beatrice Hahn of the University of Alabama, who led the study.
Published in the journal Science, the report supports other studies that suggest people caught the deadly human immunodeficiency virus (HIV) from chimpanzees, perhaps by killing and eating them.
But although Cameroon chimps appear to be responsible for the AIDS virus, the epidemic did not originate there, "We actually know where the epidemic took off. The epidemic took off in Kinshasa, in Brazzaville." Kinshasa is in the Democratic Republic of Congo, formerly Zaire.
Studies have traced HIV to a man who gave a blood sample in 1959 in Kinshasa, then called Leopoldville. Later analysis found the AIDS virus.
"So how do you get from southern Cameroon to the Democratic Republic of Congo?" Ms Hahn asked. "Some human must have done so. There is a river that goes from that southeastern corner of Cameroon down to the Congo river.”
The study suggests the virus passed from chimpanzees to people more than once, "We don't really know how these transmissions occurred," Ms Hahn said.
"We know that you don't get it from petting a chimp, or from a toilet seat, just like you can't get HIV from a toilet seat. It requires exposure to infected blood and infected body fluids. So if you get bitten by an angry chimp while you are hunting it, that could do it."
AIDS was only identified 25 years ago, the virus now infects 40 million people around the world and has killed 25 million. Spread via blood, sexual contact and from mother to child during birth or breast-feeding, HIV has no known cure and there is no vaccine, although drugs can help control it.
Harmless for Chimps
Chimps have a different version of the virus, called the Simian Immune Deficiency (SID), but whereas in people, HIV leads to AIDS and eventual death, in chimps the virus brings them no harm.
In fact, humans are the only animals naturally susceptible to HIV.
SIV has been found in captive chimps but Ms Hahn wanted to show it could be found in the wild, too. Her international team got the cooperation of the government in Cameroon and they hired skilled trackers.
"The chimps in that area are hunted. It's certainly impossible to see them. It is hard to track them and find these materials," she said. The trackers managed to collect 599 samples of droppings.
When back in the lab researchers found DNA, identified each individual chimp and then found evidence of the virus. "We went to 10 field sites and we found evidence of infection in five. We were able to identify a total of 16 infected chimps and we were able to get viral sequences from all of them," Ms Hahn said.
Up to 35 per cent of the apes in some communities were infected. Not only that, they could find different varieties, called clades, of the virus. "We found some of the clades were really, really very closely related to the human virus and others were not," she said.
Ms Hahn's study only applies the HIV group M, which is the main strain of the virus responsible for the AIDS pandemic, "It is quite possible that still other (chimpanzee SIV) lineages exist that could pose risks for human infection and prove problematic for HIV diagnostics and vaccines," her team wrote.