Why in Winter?

It’s an age-old question: “Why does flu strike in the winter?” The answers have varied: children are in close contact in school; adults spend more time together inside buildings; and people get less vitamin D from the sun during the shorter winter daylight. Some even claim that cold weather itself can give you a cold or the flu.

Researchers now have added a different explanation, backed by studies of guinea pigs exposed to the flu virus in varying air temperatures and humidity. The virus spread quickly among the test animals at 41 degrees Fahrenheit and 20 percent humidity—common winter conditions in northern climates. As the temperature and humidity rose, the transmission rate steadily declined to non-existent at 86°F, a temperature common in summer.

So what’s going on? Flu viruses, which spread though the air, appear hardier in colder temperatures. Lower humidity allows respiratory droplets containing the virus to float in the air longer, prolonging their chance to infect.

Anice C. Lowen, Samira Mubareka, John Steel, et. al., “Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature”, PLoS Pathogens 3 (Oct. 19, 2007): 1471-1476.

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