The chimp virus contains genetic material from the virus of red-capped mangabeys and also genetic material from the virus of greater spot-nosed monkeys. How did it happen? By recombination—that is, genetic mixing. A chimpanzee infected with both monkey viruses must have served as the mixing bowl in which two viruses traded genes. And when did it happen? Possibly just hundreds of years ago, rather than thousands or tens of thousands.
