Scientists have confirmed that the AIDS virus infecting more than 40 million people worldwide originated in wild chimpanzees in a remote central African forest. VOA's David McAlary in Washington tell us how the biological detectives finally tracked it down.

The evidence was obtained from chimpanzee feces from the forest floors of southern Cameroon. In the droppings, researchers from the United States, Cameroon, and Europe found clear biological signs of the closest viral relative of HIV, called SIV, a primate version of AIDS.

They detected antibodies to SIV, a type of white blood cells that immune systems dispatch to fight invading organisms.

The team leader, University of Alabama virus expert Beatrice Hahn, says further genetic analysis helped the researchers identify distinct chimpanzee communities as the sources of HIV. "That was always suspected, but no direct evidence was presented because all information was derived from captive chimps, and a lot of things can happen in captivity. I'm extending the data by showing not only that there is a natural chimpanzee reservoir, but I can further pinpoint the actual geographic location of the particular chimpanzee communities that gave rise to HIV-1," she said.

There are two chimpanzee subspecies living in Cameroon, but the one Hahn's group ties to human AIDS lives in southeastern Cameroon. It found SIV infection rates as high as 35 percent in some chimp communities.

The researchers report in the journal "Science" that the virus probably jumped to humans and became AIDS in that region, presumably when someone was bitten by a chimp or was cut when butchering an animal.

They cite previous research showing that the virus appears to have made its way south via commercial waterways like the Sanaga and Congo Rivers to Kinshasa in the Democratic Republic of Congo, where the pandemic probably began.

Hahn says blood samples show that the earliest known case of HIV was in a Kinshasa man in 1959, more than two decades before anyone knew AIDS existed. "In that part of the Democratic Republic of Congo, there is the greatest diversity of HIV-1 Group M, which is the group spreading pandemically. That suggests that probably in an urban area, Kinshasa-Brazzaville, was where we believe the epidemic started to really get going sometime around 1930, plus or minus 20 years," she said.

Hahn says previous genetic studies show that chimps got SIV from monkeys in west-central Africa.

Knowing the source of AIDS is of more than mere historical interest. The head of the World Health Organization's HIV department, Dr. Kevin De Cock, says it is important to understand its origin. For one thing, he says we could learn a lot about the biology of the disease. "If we could understand how chimpanzees harbor this infection and yet stay healthy with it, this could actually have direct biological insights that could be useful for drug development, vaccine development. But over and above all of that, from a public health standpoint, we have to know how this infectious disease came about because how do we know that similar things could not happen again?," he said.

Beatrice Hahn and her colleagues says it is possible that other SIV strains exist that could pose a risk to humans. They plan to continue analyzing chimp dung in other areas of west-central Africa to explore this question.

For now, however, Hahn says her confirmation about the source of AIDS should quell misunderstandings about its origin. "There are still some rumors circulating about how HIV-1 came about. I think it is harder to argue that the CIA [U.S. Central Intelligence Agency] manufactured it when you actually know which place in the forest it came from," she said.