Scientists have confirmed that the HIV virus really does come from chimpanzees.

Scientists long have known that nonhuman primates carry a relative of the AIDS virus, called SIV or simian immunodeficiency virus. But with one exception, it had been found only in captive chimpanzees, particularly a subspecies originating from West Africa.

It was not known how prevalent the virus was in chimps in the wild, or how genetically or geographically diverse it was, complicating efforts to pin down how it jumped from chimp to man.

Hahn's team tested chimp feces for SIV antibodies, finding them in a subspecies called Pan troglodytes troglodytes in southern Cameroon.

The team found some chimp communities with infection rates as high as 35 percent, while others had no infection at all. But every infected chimp had a genetic pattern that indicated a common ancestor, Hahn said.

There are three types of HIV-1, the strain of the human virus responsible for most of the worldwide epidemic. By genetic analysis, Hahn identified chimp communities near Cameroon's Sanaga River whose viral strains are most closely related to the most common of those HIV-1 subtypes.

Their theory is that the first human case was infected by a bite or a cut. But a bite isn't the horrible image going through my mind.