It’s difficult to imagine a world without antibiotics. They cure diseases that killed our forebears in droves, and enable any number of medical procedures and treatments that we now take for granted. Yet in 1945, while accepting a Nobel Prize for discovering penicillin, Alexander Fleming warned of a future in which antibiotics had been used with abandon and bacteria had grown resistant to them. Today, this future is imminent. Speaking to reporters last fall, Tom Frieden, the director of the Centers for Disease Control and Prevention, sounded a similar alarm: “If we’re not careful, we will soon be in a post-antibiotic era. In fact, for some patients and some microbes, we are already there.”

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