The Puzzling Origins of AIDS
Four rival theories provide some interesting lessons
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.
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 Given the lack of historical references to AIDS-like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.