Measles isn’t just catchy; it’s explosively contagious. If nobody in the population is immune, each measles-infected patient will spread the disease to twenty others, on average. The comparable number in AIDS is two to five, and in smallpox, five to seven. Meanwhile, if you haven’t had measles (or a measles vaccine) you have a 90 percent chance of contracting the disease if you’re exposed to it. In a familiar population, measles can spread quickly from child to child, until everyone is infected and immune—or, in a small percentage of cases, dead. In a fresh population that has never seen the
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