Why Is the Flu More Common During the Winter Season?

http://www.sciencedaily.com/releases/2012/12/121204162125.htm

ScienceDaily (Dec. 4, 2012) — 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.

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Their study presents for the first time the relationship between the influenza A virus viability in human mucus and humidity over a large range of relative humidities, from 17 percent to 100 percent. They found the viability of the flu A virus was highest when the relative humidity was either close to 100 percent or below 50 percent. The results in human mucus may help explain influenza's seasonality in different regions.

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The researchers found humidity could explain the seasonality of influenza by controlling the ability of viruses to remain infectious while they are in droplets or aerosols. 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, namely the concentrations of salts and proteins in respiratory droplets, and this affects the survival rates of the flu virus.