Influenza, commonly known as the flu, has distinct transmission patterns around the world. In temperate regions, influenza’s occurrence peaks during the winter season, while in some tropical regions, the disease’s occurrence tends to correspond with the rainy season. This season, the Southern United States is being hit the hardest by the flu. Have you gotten YOUR flu shot yet?

Possible explanations for the seasonality of the flu have been investigated, such as the return of kids to school, people spending more time indoors in the winter, and lower light levels that affect the immune system, but there is no agreement on them.

In order to figure this out, scientists measured the influenza A virus survival rate at various levels of humidity. This is the first time the relationship between the flu virus viability in human mucus and humidity over a large range of relative humidities, from 17% to 100%. They found the viability of the flu A virus was highest when the relative humidity was either close to 100% or below 50%–in other words, very humid or very dry (which would dry out the mucus membranes in our noses, making them less likely to "catch" a virus entering the body).

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.

The viruses survived best at low humidity, such as those found indoors in the winter, and at extremely high humidity. Humidity affects the composition of the fluid, and this affects the survival rates of the flu virus. This may help explain influenza’s seasonality in different regions.

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