Why is the Flu Seasonal?

The 2012-2013 flu season is shaping up to be a doozy. The Centers for Disease Control and Prevention have reported that during the first week of 2013, 7.3 percent of all deaths reported through their 122 Cities Mortality Reporting System were due to pneumonia and influenza, crossing their threshold for an "epidemic." Boston has declared a public health emergency, flu cases have tripled over last year in Washington, D.C., El Paso and elsewhere, and a Pennsylvania hospital has had to set up a tent outside their ER to accommodate the crush of flu patients. 

Before the dust and the snot have even settled, the director of the National Institute of Allergy and Infectious Diseases warns that this might be the worst flu season we've seen in a decade. But why is the illness seasonal?

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. The PSU team and scores of other researchers have proposed several explanations, which might work alone, simultaneously but separately, or in combination with each other:

Weather and climate

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 degrees), dry (20 percent humidity) conditions and declines as temperature and humidity rise (at 86 degrees or 80 percent humidity, it wasn't transmitted at all).

Human behavior

Fall brings 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

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.