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The Puzzling Origins of AIDS

Four rival theories provide some interesting lessons

Jim Moore

Shortly after the 1983 discovery of the human immunodeficiency virus (HIV), the pathogen responsible for AIDS, investigators became aware of a strangely similar immune deficiency disease afflicting Asian monkeys (macaques) held in captivity in various U.S. research labs. Soon, virologists identified the culprit: a simian immunodeficiency virus (SIV) that is found naturally in a West African monkey species, the sooty mangabey (Cercocebus atys), but is harmless to that host. This virus, denoted SIVsm, is genetically similar to a weakly contagious form of the AIDS virus that is largely restricted to parts of West Africa, HIV-2, and thus is considered its likely precursor. More recent work has shown that the closest relative of the primary human immunodeficiency virus (HIV-1) is another simian immunodeficiency virus, one carried by chimpanzees (SIVcpz).

After comparing the SIVs in chimpanzees and sooty mangabeys with HIV-1 and HIV-2 strains, investigators concluded that there must have been multiple transmission "events" from simians to humans—at least seven for HIV-2 (some of which are known from only a single person who lives near mangabeys carrying a uniquely similar SIV) and three for HIV-1, the virus now infecting some 40 million people worldwide.

Postcards from early 20th-century AfricaClick to Enlarge Image

How did SIVcpz and SIVsm cross over into humans and become pathogenic? Given the lack of historical references to AIDS-like disease in Africa prior to the mid-20th century, as well as its absence previously in the New World (which imported some 10 million African slaves during the 16th through 19th centuries), that transfer appears to have happened relatively recently—exactly when is a point of considerable debate. And why did two distinct simian viruses with which humans have apparently coexisted for centuries, or even millennia, suddenly pass into humans multiple times within a few decades?

The answers to these questions have been slow in coming, despite the considerable efforts of molecular biologists to understand the nature and evolution of primate immunodeficiency viruses. I am not one of those molecular biologists; rather, I became a player in the field of AIDS-origin research through my interest in chimpanzee socioecology. Although I am partial to a theory I helped to fashion for why AIDS emerged when it did, with time it might become clear that a competing idea better accounts for genesis of the epidemic. Or perhaps the answer will prove to lie with some complex combination of factors that no single explanation presently encompasses. Whatever the case, the solution almost certainly will come from one or more of four competing theories.

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