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ut across the city. In October 1918, a > The New York Times, the shooter "was charged wear surgical masks, the odds drop to 30 percent or less. And if they both wear N95s or KN95s, the odds drop to 0.4 percent or less. (In case you're wondering, the 95 indicates that, in testing, the mask's material blocked 95 percent of very small particles blown with enough force to simulate a human doing hard work. On an N mask, the elastic straps go around the back of your head, while the straps on a KN mask go around your ears.)* Of course, every chain is only as strong as its weakest link, and masks can only stem an outbreak if enough people wear them. In America, unfortunately, resistance to wearing masks is almost as old as masks themselves. During the 1918 flu epidemic, just a few years after Wu's breakthrough finding, several U.S. cities adopted mask mandates. In San Francisco, anyone who failed to wear a mask in public could be fined or put in jail.30 Protests broke out across the city. In October 1918, a "mask slacker" beat a health inspector with a bag of silver dollars for insisting that he cover up. The health inspector pulled out a pistol and shot him.† * Masks like the N95 and KN95 are, confusingly, also referred to as "respirators." I'll just call them masks, to avoid confusion with the machines that help patients breathe. † Both men survived. According to the The New York Times, the shooter "was charged with disturbing the peace, resisting an officer and assault. The inspector was charged with assault with a deadly weapon."