Where Does the Flu “Go” When It’s Not Flu Season?

Updated: Feb. 07, 2023

When flu season is looming, experts recommend a flu shot—but why does a virus have a season? Here's what the experts say.

When the flu season intensifies, you have to wonder what happens to flu viruses the rest of the year? As it turns out, they migrate—not unlike birds. Though, while you’re waiting for it to go away, follow these 9 ways to prepare for the worst month of flu season. Plus, our guide to bird AKA avian flu.

“The flu season can begin as early as October in the United States and ends in March, with a peak in February,” explains Kola Dushaj, MD, an internal medicine doctor at CareMount Medical in Cortlandt Manor, New York. Although there are fewer cases after March, the season can still occur into May, according to the Centers for Disease Control and Prevention. But the flu season also depends on your location. “In the southern hemisphere, where winter coincides with our summer months, the flu season can run from June to September,” says Dr. Dushaj.

So in the same way birds fly south for winter, the flu virus migrates—though in the opposite direction. “Studies have been done to analyze the migration of seasonal influenza A viruses,” says Dr. Dushaj. “After studying genomes from both southern and northern hemispheres, research reveals that cross-hemisphere migration frequently occurs and it can occur in both directions.” Plus, thanks to our modern modes of international travel and delivery of goods, says Dr. Dushaj, the flu virus can easily hitch rides around the world.

How does the flu stay alive?

A woman suffering from coldRawpixel.com/ShutterstockThe only way for viruses to survive and spread is to infect people (and animals). “Outside of a host, the virus lifespan is shortened. The influenza virus can live on some surfaces for up to 48 hours,” explains Dr. Dushaj. Because research suggests that colder weather favors a virus’s ability to survive, it travels in hosts to hospitable climates—winter—where it can more easily spread, he says. And that’s just one of the many secrets the flu virus doesn’t want you to know.

The flu’s migration (and season) is also based on humidity and temperature. “The flu virus tends to infect more people during cold and dry weather,” says Dr. Dushaj. Other conditions that encourage the spread of a virus may be staying indoors with sealed windows and recirculated air—as we do in winter. This increases the risk of inhaling the same air as someone who may have the flu, he says.

There are other theories on why the flu is worse in winter, says Dr. Dushaj: “Less sunlight exposure in the winter months leads to lower levels of vitamin D and melatonin [in your body].” Being low in these substances may undermine your immune system and your ability to fight off infection, he explains. Stay safe by getting a flu shot, he recommends. You may not know immediately if you have a cold, the flu, or something else, but here are the clear symptoms it’s the flu.