Cases of the flu peak in winter in the U.S. But why? A new study suggests it’s not the heat, but the humidity. Or lack thereof. Because in temperate regions, the influenza virus fares best when the weather is dry. That’s according to work published in the journal PLoS One. [Wan Yang, Subbiah Elankumaran and Linsey C. Marr, Relationship between Humidity and Influenza A Viability in Droplets and Implications for Influenza’s Seasonality]

Scientists have long debated why flu erupts when the days grow chilly. Is it that we spend more time cooped up together indoors? Or is there something about the virus that likes it cold and dry? To find out, researchers suspended influenza virus in a solution that mimics human mucus. They incubated this infectious soup at different humidities and measured viral survival.

And they found that at low humidity, the fake mucus dries up and the virus does just fine. But when the humidity tops 50%, the droplets only partially evaporate, leaving behind a solution that’s too salty for the virus to thrive.

Interestingly, the virus does well again when the humidity reaches 100 percent, evaporation stops and the salinity of the mucus bath is juuust right. That could explain why the flu prefers to hit the tropics in rainy season. And why you should always keep your nose clean, but moist.

—Karen Hopkin

[The above text is a transcript of this podcast.]