WASHINGTON (AP) — The ancestry of HIV has been traced to two strains of virus found in monkeys in Africa.

The viruses probably passed into chimpanzees when the apes ate infected monkey meat, researchers say.

Earlier studies have shown that HIV1, the virus that causes the most common form of human AIDS, originated from a simian immunodeficiency virus, or SIV, that is found in chimpanzees. But how chimps came to have SIV has been a mystery.

American and English researchers analyzed the genetic pattern of a number of SIV strains in African monkeys and concluded that at least two strains found in the red-capped mangabeys and in the greater spot-nosed monkeys in south-central Africa combined to form the type of SIV now found in African chimps.

It was this form of SIV that spread into the human population to start the HIV1 epidemic that has killed millions of people, researchers report in the journal Science.

“The recombination of these monkey viruses happened in chimpanzees and the chimp transmitted it to humans on at least three occasions,” said Frederic Bibollet-Ruche, a virologist at the University of Alabama, Birmingham, and a co-author of the study. “The transfer between chimps and humans probably happened before 1930.”

Bibollet-Ruche said that three types of HIV1, called M, N and O, probably were transmitted from chimps to humans decades ago. A second type of AIDS, called HIV2, is known to have been transmitted from the sooty mangabey in West Africa to humans directly, without going through the chimp.

Monkeys and chimps both represent a reservoir of SIV viruses that could, in theory, be spread to humans, forming a new type of immunodeficiency disease, he said.

The viruses most likely spread from species to species when chimps eat monkey meat and hunters in Africa eat chimp meat, Bibollet-Ruche said.

“Chimps are known to hunt and eat whatever monkey species they can catch,” he said.

As for humans, Bibollet-Ruche said “it is not such a good idea to hunt and eat monkeys. There is a risk for humans to come into contact” with a new form of HIV.

Bibollet-Ruche said the genetic studies suggest that lower monkeys first became infected with SIV 100,000 years ago or even earlier.

SIV was passed to chimps after the animals split up into different subspecies living as separate bands in West Africa and in southern and central Africa. He said the easternmost subspecies of chimps is infected with SIV, but the virus has not been found in chimp tribes in West Africa.

Although SIV can infect chimps and the lower monkeys, the virus does not cause disease in those animals. Bibollet-Ruche said that the virus attacks the white blood cells, called CD4 cells, but it does not make the animals sick or cause a decline in their white blood cells.