It was October 31, a balmy day in Brooklyn, and Alexander Arroyo was walking around his neighborhood dressed as an octopus, pushing his 2-month-old daughter in a carriage, as his wife chased their toddler through the after-school Halloween trick-or-treat crowd. As the family filled their bags with candy, Arroyo’s phone rang and he stopped to answer it, trying to hear over the din of excited children. Arroyo is the director of the pediatric emergency department at one of the biggest hospitals in Brooklyn, Maimonides Medical Center, and two days earlier, a 15-month-old girl had come to the ER with a fever and a rash. He’d been waiting for a call to confirm the diagnosis, and this was it. The test had come back positive: The girl had measles.

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