Hardly any doctors still practicing can remember life before antibiotics, when people were routinely hospitalized for common infections, and the threat of deadly Staphylococcus shadowed even the simplest surgery. But infectious-disease specialists like Brad Spellberg of UCLA’s David Geffen School of Medicine have been reading up on those days because of a growing fear they are not all in the past. Wealthy countries take for granted the triumph of science over bacteria, but increasingly doctors are coming up against infections that can be quelled only by the most powerful antibiotics known to medicine—or by none of them. “It’s already happening,” says Spellberg, to the tune of roughly 100,000 deaths a year from antibiotic-resistant infections in the United States alone. “But it’s going to become much more common.” Imagine a world in which antibiotics resemble chemotherapy drugs—producing toxic side effects and unpredictable outcomes instead of the guaranteed cures we have come to expect—and you can understand what keeps Spellberg awake at night.

In the future, historians of science may debate whether victory over bacteria was ever within our grasp. But it seems almost certain that the 60 or so years after penicillin came to market will eventually be viewed as just an interlude in the eternal war between us and them. We are multicelled animals of astonishing complexity and delicacy, moving through a world in which they vastly outnumber us. They are single-celled organisms so primitive they lack even a nucleus, marvelously adapted to multiply inside us—under the right circumstances, to consume our flesh and poison us with their waste. For a few decades we gained the upper hand through the use of antibiotics, natural substances that are as toxic to germs as germs are to us. But our ingenuity is in a desperate race against their ability to reproduce.