Why Is the Flu Seasonal?

The current flu season is shaping up to be a doozy: According to the Washington Post, the hospitalization rate is three times higher for flu than for COVID-19 as of January 2025. But why does the flu even have a season?

1. Influenza viruses travel the globe.
2. Weather and climate influence influenza.
3. Human behavior spreads the virus.
4. Human physiology leaves us susceptible.

Influenza viruses travel the globe.

In the temperate regions of the world, the flu tends to strike hardest in the autumn and winter. The conventional wisdom used to be that the influenza viruses either went into a dormant state or persisted at very low levels during the summer months before flaring up again. Scientists have since figured out that, instead of simply lying low during their “off season,” the viruses also go globe-trotting, and get transmitted throughout populations all over the world.

In 2007, researchers from Pennsylvania State University and the National Institutes of Health found that the influenza A virus uses its summer travels to meet exotic viruses in tropical areas (which experience year-round flu virus activity), swap genetic information with them, and then roll back into town with enough genetic differences to fool our immune systems. Scientists are still working out what exactly triggers the reintroduced viruses to infect people during the fall and winter.

Weather and climate influence influenza.

Influenza viruses do very well in cold winter temperatures and the dry air that goes with them. They can survive longer in dry air than moist air, and hold out longer on exposed surfaces (counters, doorknobs, keyboards, etc.) when they're cold. For humans, dry air means dehydrated mucus and drier nostrils and airways, which could make it easier for the viruses to make themselves at home once they're passed to us. A study on guinea pigs at the Mount Sinai School of Medicine showed that the transmission of influenza is enhanced in cold (41°F), dry (20 percent humidity) conditions and declines as temperature and humidity rise (at 86°F or 80 percent humidity, it wasn't transmitted at all).

Human behavior spreads the virus.

Fall and winter bring a new school year and cooler outside temperatures, and more people spend more time indoors in close contact with each other, giving the viruses an easy route for transmission between hosts. Even in tropical regions that don't have a winter and where flu occurs throughout the year, illness tends to spike during the rainy season when people spend time together indoors.

Human physiology leaves us susceptible.

Thanks to all that time indoors and the short winter days, our bodies’ Vitamin D decreases in the winter. This decrease, or any number of other seasonal tweaks to our immune systems, could leave us more susceptible to the virus for a few months out of the year and act as a “seasonal stimulus” for flu infection. 

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A version of this story was published in 2013; it has been updated for 2025.