A lot of good timelines on this site have alternate epidemics/diseases and they do shape history by killing millions of people, so even though it's more of a biological question it seems fair to post on a what-if site primarily focused on history: Could cats have given people AIDs? Why didn't they? Feline immunodeficiency virus is in the same genus as HIV. It's generally spread in cats through saliva in deep bite wounds (because cats are nasty buggers to each other), which cats are just as physically capable of inflicting on humans as they are on other cats. Human beings have been exposed to cats for thousands of years while living sedentary existences in densely-populated places. We got HIV from nonhuman primates. Is it theoretically possible to for somebody to have gotten AIDs from a cat? What would FIV turned into *HIV be like if it's possible? What effects would it have had on a preindustrial society?