Last month, the chief medical officer of Britain called antibiotic resistance a “ticking time bomb” and a threat as dangerous as global warming. In Europe alone, 25,000 people die every year from antibiotic-resistant bacteria — and that’s only counting the infections that were picked up in supposedly sterile hospitals.

For more than 80 years, antibiotics have been nothing short of miraculous. Capable of killing bacteria without killing people, they’ve turned grave illnesses into mere annoyances, providing doctors with license to shoot first and ask questions later, and do so safely — or so we thought. Broad-spectrum antibiotics are powerful fighters with one flaw: unable to smite every bacterium, those immune to their wrath thrive. With every ear infection we treat, and every healthy cow prophylactically dosed with antibiotics (which also helps fatten the animals), we make these drugs less useful for future generations.

Scientists all over the globe are in a race with evolution, scrambling to understand the underlying mechanisms of antibiotic resistance and to discover new ways to fight bacteria. We must diversify our methods for treating bacterial infections and simultaneously reduce the amount of antibiotics we use, says Brad Spellberg, an infectious-disease specialist at U.C.L.A. This has led to a renewed interest in treatments from a world before penicillin.

Ruth Greenwood, a friend’s grandmother, knows the cruelty and desperation of this world well. In 1934, at age 9, she came up with a clever but shortsighted plan to get out of school: roll around in a patch of poison ivy. Greenwood succeeded in getting an itchy rash (and skipping class), but as the days went by, the rash became something much worse. Her legs swelled and broke out in a mass of tender, crimson sores that wept blood and pus. At night, the fluids soaked through her sheets and dried, fusing the bedding to her skin. Greenwood remembers being scooped up in the morning, sheets and all, and put in the bath to soak until the cloth could be peeled away from her flesh.