Why Is Flu Season in Winter?

It has to do with the dry winter air, says new research

Why is flu season in the winter? Why, on the darkest, coldest nights of the year, do you also have to be laid out by a fever and runny nose and a body where your everything hurts?

Maybe the short, cold, blustery winter days keep us inside and the lack of sunlight weakens our immune systems? Or the flu starts to spreads when all the grubby kids in school (or university students in massive dorms) get back together and start touching each other. Or hiding out from the cold just puts us all in closer contact, giving the flu a chance to spread. All these and some other “possible explanations for the seasonality of the flu have been investigated…but there is no agreement on them,” says a release from Virginia Tech.

Based on new research, scientists led by Virginia Tech graduate student Wan Yang found that the seasonality might have more to do with the weather than any of the vagaries of human behavior. They found that the survival rate of influenza viruses—the ones that cause the flu—soars outside of the body when the humidity is below 50 percent humidity or when it is close to 100 percent.

In temperature regions like the continental U.S., winter usually brings cold, dry air, and a dip in relative humidity. Dry air makes sneeze droplets evaporate quickly, allows the viruses to hang around in the air.

At low humidity, respiratory droplets evaporate completely and the virus survives well under dry conditions. But at moderate humidity, the droplets evaporate some, but not completely, leaving the virus exposed to higher levels of chemicals in the fluid and compromising the virus’ ability to infect cells.

Blegh. Let’s just be happy that it’s spring.

More from Smithsonian.com:

The Flu Hunter
Can You Give the Flu To Your Dog or Cat?

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