The glory of cities is to serve as places of interaction between people and economies. Yet throughout history—from Roman times to the present—this advantage has also entailed exposure to deadly contagions. As Marc Riedl, a specialist in respiratory disease at UCLA, puts it: “Megacity life is an unprecedented insult to the immune system.”

Today’s coronavirus pandemic reflects these patterns, concentrating, at least initially, in densely populated regions, such as Wuhan, Madrid, and around Milan. In the United States, the vast majority of cases to date are occurring in the densest, most globalized regions, such as Seattle, San Francisco, and, in particular, greater New York. Cases have been far less prevalent, so far, in the vast middle of the country—except for New Orleans and Detroit—and in rural areas, where people have less daily contact.

Yet density is not a death sentence. Taiwan and South Korea—which have denser cities—and Singapore, roughly the density of New York City, have largely avoided the worst of the outbreaks. In all three countries, health officials took decisive and early steps to control travel and identify those who might be susceptible to the disease.

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