The cross-species infection can originate on farms or markets, where conditions foster mixing of pathogens, giving them opportunities to swap genes and gear up to kill previously foreign hosts (i.e. you). Or the transfer can occur from such seemingly benign activities as letting a performance monkey on some Indonesian street corner climb on your head. Microbes of two varieties can even gather in your gut, do some viral dancing, and evolve to morph you into a deadly, contagious host.
Diseases passed from animals to humans are called zoonoses. There are more than three dozen we can catch directly through touch and more than four dozen that result from bites.
But disease-carrying parasites are not picky about hosts. Human diseases can decimate animal populations, too, from such well-meaning activities as ecotourism.
SOURCE: CDC; WHO; National Archives; LiveScience reporting