Fortunately, life tends to lean toward cooperation, happily forming alliances to advance a shared cause. That, presumably, is why our planet is completely coated with living matter. And that is how, some millions of years ago, bacteria and animals struck up a deal. In return for a moist bed and a warm buffet, beneficial bacteria took up the job of defending us against the madly proliferating pathogens in the world. It takes a germ to fight a germ.