The HIV-1 group O virus, though, is responsible for about one percent of current human HIV cases in Cameroon. Until the latest study, group O had not been found in nonhuman primates.
Peeters says it is still too early to say whether western lowland gorillas are a reservoir species or merely a carrier of the virus.
But she believes chimpanzees could have infected gorillas with the group O virus in the distant past.
More than 30 primate species are known to carry SIV viruses, the study authors note.
But—unlike HIV-positive humans—chimps, gorillas, and other primates that carry SIV do not appear to suffer illness.
Peeters says a goal now "is to find out how [HIV] moved—when, how, and which animal transmitted it to humans."
Such findings could better explain the processes that drive some viruses to leap from animals to humans and cause global disease pandemics, while other outbreaks remain restricted to small populations, Peeters says.
"What we would like to know now is, [is] this virus circulating in chimpanzees and gorillas? How [have these] been transmitted from one species to another? What happens amongst humans when there is cross-species transmission?"
Biologist Eddie Holmes studies virus evolution at Pennsylvania State University in University Park.
Holmes described the new study as "extraordinary."
"I guess I ruled out gorillas as a carrier of HIV," he said, adding that "it is amazing to learn that there is a new primate carrier of HIV."
He thinks the findings are also significant because they further illuminate the diversity of the HIV virus in natural populations and the processes by which these viruses jump species.
"If you look at the rise of HIV, there's been a jump from chimpanzees to humans," Holmes said. He attributes the transfer of HIV to logging and to trading wild animal meat, both activities that bring humans into close contact with ape and monkey flesh. (See a "bush meat" photo gallery—warning: graphic images.)
Holmes says growing human interaction with wild species—as a result of changing land use, deforestation, global travel, war, famine, and the expansion of cities—will provide more opportunities for new viruses to emerge.
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