Of the four identified groups of human AIDS viruses—HIV-1 M, N, O, and P—only M and N had been traced to their source. Until now. Researchers from institutions around the world, including the University of Pennsylvania School of Medicine, are reporting that the origins of groups O (the second-deadliest strain, having infected 100,000 West Africans) and P (the rarest, found in only two) have now been identified. Unlike the deadliest group M (infecting at least 40 million humans) and group N (seen in only a couple dozen), both of which leaped from chimpanzees, groups O and P crossed over from western lowland gorillas in southern Cameroon. However, these simian immunodeficiency viruses (SIVs) both leaped from chimps to gorillas, so "the gorilla virus is basically a chimp virus in gorillas," one study author says.
Researchers took on the fetid task of analyzing the feces of some 3,000 gorillas spread out over several western and central African countries to pinpoint the source. Though the puzzle pieces of the origins of HIV are coming together, several mysteries remain, reports the Chicago Tribune. Among them: why group O didn't spread more rapidly between chimps and gorillas, who share a habitat; less than 2% of this source population of gorillas in southern Cameroon were infected with gorilla SIV. The researchers also plan to study whether gorilla SIV results in a disease resembling AIDS, as it does in chimps and humans, and also whether SIV is at least partly behind the decline of this population of gorillas. (Scientists first discovered in 2009 that SIV in chimps can lead to low T-cell counts just like AIDS in humans.)