Why Flu Epidemics Work Differently in Big American Cities

By Rafi Letzter

Flu epidemics in small towns are bad, but at least researchers understand them. A new paper published today (Oct. 5) in the journal Science shows that dense urban centers in the United States have lost a lot of the natural defenses that keep the flu from rampaging year-round through the population. And that causes the flu to behave in ways scientists are just beginning to understand.

Under normal circumstances, the researchers wrote, the flu is contained to the colder, dryer parts of the year. That’s because the virus spreads through the air; when a sick person coughs and sneezes and otherwise expels the flu into the air, it can survive long enough to infect someone else nearby. But the humid, warmer months of the year are bad for the virus. It can’t survive exposure to that wet air as long, and struggles to spread from person to person. So flu peaks in winter epidemics and largely subsides in the summer.

In America’s big cities though, the researchers found, this seasonal, natural flu regulation has broken down. People are so frequently packed so closely together that the flu never really loses its ability to spread through the population. All that humidity in the air is less of a problem for the virus when it has to travel just a few inches to reach the next person.

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