Bovine spongiform encephalopathy
The Fore of New Guinea were nearly wiped out by kuru, which bears a striking resemblance to B.S.E.; they spread it among themselves by ritually eating the brains of their dead kin. Biologists think that evolution probably selected against cannibalism as a way to avoid such infections (among other things). Many animals' instinctive aversion to their own feces and to the carcasses of their species may represent similar strategies to avoid infectious microbes and parasites. Through natural selection, animals have developed what amount to a set of hygiene rules that function much like taboos. One of the most off-putting things about factory farms is how cavalierly they flout these evolutionary rules, forcing animals to overcome deeply ingrained aversions. For their instincts we substitute antibiotics."