Scientists believe they have found a "missing link" in the evolution of the virus that causes AIDS.
It bridges the gap between the infection that does no harm to most monkeys and the one that kills millions of people.
That link is a virus that is killing chimpanzees in the wild at a disturbingly high rate, according to a study in Thursday's journal Nature. Chimpanzees are the first primate besides man shown to get sick in the wild in significant numbers from a virus related to HIV. Chimps are also man's closest relative among primates.
And chimps are already endangered.
But the discovery of the disease killing chimps may help doctors come up with better treatments or a workable vaccine for humans, experts said.
The monkey version of the virus that causes AIDS is called simian immunodeficiency virus (SIV), but most apes and monkeys that have it show no symptoms or illness.
So "if we could figure out why the monkeys don't get sick, perhaps we could apply that to people," said study lead author Beatrice Hahn, a professor of medicine at the University of Alabama at Birmingham.
The nine-year study of chimps in their natural wild habitat at Gombe National Park in Tanzania found chimps infected with SIV had a death rate 10 to 16 times higher than uninfected chimps.
Chimp disease is close relative to HIV
And necropsies of dead infected chimps showed unusually low counts of T-cell white blood proteins that are just like the levels found in humans with AIDS, Hahn said in a phone interview.
And when scientists looked at the particular strain, they found that it was the closest relative possible to the virus that first infected humans.
"From an evolutionary and epidemiological point of view, these data can be regarded as a 'missing link' in the history of the HIV pandemic," AIDS researcher Dr. Daniel Douek of the National Institute of Allergy and Infectious Diseases wrote in an email. Douek was not involved in the Nature study.
Monkeys and apes — except for chimps — seem to survive the virus because of some kind of evolutionary adaptation, probably on the cell receptors, Douek wrote. The infection of chimps is more recent so they haven't adapted, he wrote.
Hahn said chimps and people probably caught the virus the same way, by eating infected monkeys. And they both spread it the same way, through sexual activity.
Many factors are causing Africa's chimp population to dwindle, said study co-author Michael Wilson, a professor of anthropology at the University of Minnesota and former director of field research at the Jane Goodall Institute in Tanzania. Hunting, loss of habitat and disease are decreasing chimp numbers and it's hard to figure out how much of a factor SIV is, he said.
"It is a concern," Wilson said. "The last thing these chimps need is another source of mortality."
Wilson, who spent years observing chimps in Tanzania as part of the study, said that when researchers realized the virus was fatal and they knew which chimps were infected, it became hard to watch some of their activities in the wild.
Wilson recalled wanting to warn one female chimp: "Don't mate with those guys."
"But of course I can't do that."