For scientists that study disease, one unnerving fact is always present: somewhere on the planet, there’s an organism or organisms — likely bacteria or a virus — that could kill tens of millions if it started to spread among people.

Pathogens we’ve never seen before lurk around the world, potentially living in bats, mosquitoes, or ticks, and waiting to jump to humans. Existing diseases mutate, becoming deadlier or more contagious. People are even growing more capable of modifying deadly diseases on their own. Read more.