A monkey virus similar to HIV is endemic in wild gorillas in Africa and was probably transmitted to them by chimpanzees, researchers said on Wednesday.
About 40 million people worldwide are living with HIV/AIDS.
The origins of two of the three strains of the virus in humans have been traced back to monkeys in Africa infected with simian immunodeficiency virus (SIV) but the source of the third has been unknown, until now.
"It is the first time that someone has done a survey among wild gorillas to see whether they were infected with an SIV," said Martine Peeters, a virologist at the University of Montpellier in France.
"We showed they were infected and moreover they are infected with a virus that is closely related to HIV-1 and a particular variant O," she added in an interview.
HIV is thought to have been passed on to humans when they slaughtered infected chimpanzees for food. About 25 million people have died of HIV/AIDS since the virus was identified a quarter of a century ago.
There are three strains or groups of HIV -- M, N and O. Group M is the most common strain and has spread around the globe. Strain N is linked to few cases in Cameroon and group O represents about one percent of HIV/AIDS cases in Cameroon and surrounding countries.
"It is only there that we find it," Peeters explained, referring to the O strain.
She and her colleagues collected and examined hundreds of droppings from wild gorillas and chimpanzees living in remote forest areas in Cameroon. The animals are still hunted for food and medicines.
An analysis of the samples showed the gorillas were infected with a strain of SIV related to the O group. The infected gorillas lived nearly 400 kilometers (250 miles) apart so the scientists believe it is likely SIV infection is endemic in the animals.
"We have discovered it in gorillas but we think the primary reservoir are still chimpanzees. We think chimpanzees transmitted it to gorillas but we don't know who transmitted it to humans -- the gorilla or the chimp," Peeters, who reported the findings in the journal Nature, said.
How the animals acquired it is also a mystery because gorillas are vegetarians and encounters with chimpanzees are thought to be rare.
Knowing the origin of the HIV and that is crossing species is important for understanding what happens to the virus when it jumps species.