11 (sometimes) deadly diseases that hopped across species
Bacteria and viruses that are deadly to one type of creature can evolve quickly to infect another. While SARS-CoV-2 (which causes COVID-19) is the latest example, a host of infectious and deadly diseases have hopped from animals to humans and even from humans to animals.
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 infect (and sometimes kill) previously foreign hosts. 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 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.