Viruses and bacteria which are deadly to a single kind of creature can evolve fast to infect another. Even though SARS-CoV-2 (which induces COVID-19) is the latest example, a host of infectious and lethal diseases have hopped from animals to people as well as from humans to animals.
The cross-species disease can originate on markets or farms, where states cultivate mixing of pathogens, giving them opportunities to exchange genes and equipment up to infect (and occasionally kill) formerly foreign hosts. Or the transfer can happen from such seemingly benign activities as letting a functionality monkey on a Indonesian street corner scale in your mind. Microbes of two kinds can even collect on your gut, do a bit viral dancing, and evolve to morph you in contagious host.
There are more than three dozen people could grab directly through touch and more than four dozen which result from snacks. But disease-carrying parasites aren’t picky about hosts. Human diseases can decimate animal inhabitants, also, from such well-meaning activities as ecotourism.