Some groups have even suspected that homosexual men were deliberately infected, but most scientists believed it must have come from apes or monkeys. Humans are the only creatures that can get HIV, which stands for human immunodeficiency virus. Nonetheless, only three cases of chimpanzees infected with SIV had been documented. But it came from a subspecies of chimp known as Pan troglodytes schweinfurthii. This virus infects a primate species that is 98 percent related to humans.
Many viruses come from animals.
Flu, for example, comes from ducks and pigs. Often the virus does not make its natural host, or reservoir, sick. It must undergo genetic changes to infect another species such as humans, changes that can cause illness. Hahn's team found evidence of such genetic changes in their chimp SIV, known specifically as SIVcpz. That is just where this subspecies of chimp lives.