According to new research, simian immunodeficiency virus (S.I.V.) has been in monkeys for millennia, potentially putting humans at risk for the last 32,000 years and possibly much longer.
And yet, for all that time, humans didn't get sick in mass. Only in the 20th century did H.I.V. become a global scourge that has claimed 25 million lives.
Why?
According to the New York Times, for as long as monkeys have had S.I.V., humans who have butchered them have put themselves at risk of infection from a mutated form. But because the infected people in Africa were fairly isolated, the chances for an epidemic were small. That changed, some theorize, with the explosive growth of African cities and wide spread use of cheap syringes.
But the reality is, no one really knows for sure.
The new research does help explain why monkeys who have S.I.V. do not get sick from it - they have had tens of thousands of years to adapt.
In order to track S.I.V. back in time, researchers led by the University of Arizona and the Tucson and Tulane National Primate Research Center looked at the DNA of 79 monkeys from Bioko, a volcanic island off the coast of West Africa.
According to the New York Times, the island was cut off from the mainland 10,000 years ago and six species of monkeys have developed exclusively there. Four of them had S.I.V. That meant the virus was at least 10,000 years old. Scientists then measured how fast the virus mutates and calculated its age at between 32,000 and 78,000 years, the Times reports.