Even bacteria get sick. Tiny though they are, bacteria can be infected by even tinier viruses known as phages. Like tiny hypodermic needles, phages inject their genetic material into their bacterial hosts, turning them into factories for making more phages. The host usually dies in the aftermath. But some bacteria have turned these enemies into their allies. By adding the viruses’ DNA into their own genomes, they have become superbugs, able to tolerate harsh environments and shrug off antibiotics.

Once phages have injected their genes into a bacterium, they can make copies of themselves in two ways. The first is a brutish approach. The genes commandeer the host, using it to manufacture new viruses that eventually burst out of the