Acquired immunodeficiency syndrome (AIDS) received its utilitarian name in 1982, a year after U.S. doctors recognized an epidemic of pneumonias, rare cancers and assorted bacterial infections among mostly male, mostly young and mostly previously healthy adults. The next year French researchers isolated the cause of the immune system collapse that defined the syndrome: a virus that selectively infects and destroys immune cells themselves.
The human immunodeficiency virus (HIV), which today resides in more than 30 million people and seemed to come out of the blue in the early 1980s, is now known to have been infecting humans for at least a century. Recent studies of preserved tissue samples show HIV present in the former Belgian Congo in 1959, in Haiti by 1966 and possibly in the U.S. as early as 1969. The historic specimens also let scientists calibrate “molecular clocks” to trace the evolution of the virus back to its first appearance in humans.
Those analyses place the emergence of the most widespread HIV strain, known as group M, in southern Cameroon around 1908. Its ancestor was likely a virus that has been infecting West African chimpanzees since 1492, according to another recent molecular clock study. If so, many rural people were surely exposed to simian immunodeficiency syndrome (SIV) over the centuries through live chimps or in bush meat before the infection caught hold in the human population. Scientists are consequently keen to figure out what allowed “successful” SIV strains to adapt to our species and begin spreading as HIV.
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AIDS researchers are also intensively studying the behavior of SIV in its native host because although the simian virus is nearly identical to HIV, in wild chimps it is generally benign. The immune cells of our closest primate cousins get infected, too, but eventually manage to rally and reconstitute their numbers. The origin of the devastating syndrome that is AIDS therefore lies in some combination of minute changes in HIV itself—and the human body’s responses to it—and remains a mystery.