Hunting for - and eating - primate "bushmeat" is exposing humans to a form of virus carried by apes and monkeys, experts say.
Chimpanzees carry viruses which can jump to humans
The effect on humans of simian foamy virus (SFV) is not yet known - but it is thought that HIV originally passed to humans in the same fashion.
Johns Hopkins University experts say the only way to stop the virus's spread in humans is to restrict hunting.
The research is published in The Lancet medical journal.
The hunting and butchering of wild primates - including monkeys, gorillas and chimpanzees - infected with simian immunodeficiency virus (SIV) is thought to have sparked the HIV pandemic two decades ago.
But it seems that SIV, SFV and related retroviruses cannot jump the species barrier unless humans come into direct contact with infected blood tissue or fluids.
The latest research focused on 1,800 people from nine rural communities in Cameroon, of whom around 1,100 reported they had been exposed to blood or body fluids of primates from hunting.
Of these, 10 people were found to have developed antibodies to SFV.
Further genetic analysis revealed that the infections had come from different animals.
Lead researcher Dr Nathan Wolfe said: "Our findings show that retroviruses are actively crossing into human populations, and demonstrate that people in central Africa are currently infected with SFV."
He said that it was possible that a human form of SFV might emerge, in the same way that scientists believed SIV morphed into HIV.
"Contact with non-human primates, such as happens during hunting and butchering, can play a part in the emergence of human retroviruses - and the reduction of primate bushmeat hunting has the potential to decrease the frequency of disease emergence," he said
In an accompanying commentary in the Lancet, Dr Martine Peeters, from the Institut de Recherche pour le Dévelopement, Montpellier, France, warns that cross-species "zoonotic" diseases are "among the most important public health threats facing humanity".
She said foamy viruses have not been linked with any disease in humans, and there was no evidence that they could be passed between individuals.
However, she said very little was known about what possible effect SVF might have on humans, as few had documented instances of human infection.
And she warned that the possibility that particular strains of the virus might cause disease - possibly after a long incubation period - could not be ruled out.