How the Flu Spreads — and Ways to Protect Yourself

Coughing, sneezing, close contact: These are among the ways the flu spreads from one person to another, causing unpleasant symptoms like cough, sore throat and muscle aches to set in anywhere from one to four days after infection — and in some cases making people contagious before they even know they are sick.

This year, experts aren't just worried about the toll of flu season, which typically peaks between December and February in the United States. They're also concerned about the possibility of a “twindemic” — the overlap between the ongoing COVID-19 pandemic and the upcoming flu season — which, they say, makes public awareness about how the flu spreads and how to stay healthy more important than ever.

How the flu spreads

"The major mode of transmission with influenza is by droplet,” says Keith Roach, M.D., an internist at Weill Cornell Medicine and NewYork-Presbyterian. When someone with the flu coughs, sneezes or even talks, he or she produces respiratory droplets containing the virus that can land in the mouths or noses of others nearby, making them sick.

According to the Centers for Disease Control and Prevention (CDC), this kind of person-to-spread spread is possible at distances of up to 6 feet — and adults who get the flu can infect other people starting one day before they develop symptoms.

Direct contact is the second way flu spreads, Roach says, like shaking hands with an infected person who just rubbed his own nose or touched his eyes (or touching a doorknob or other surface he touched) and then touching your own mouth or nose.

But not all exposure to the flu virus is created equal. Experts say that contact with infected surfaces is generally less of a concern than person-to-person spread, and even then, variables remain.

"Someone's risk of getting infected with either flu or SARS-CoV-2 is related to some extent to the concentration of virus that you're exposed to [multiplied by] the amount of time you're exposed to it,” says Dean Winslow, M.D., an infectious disease physician at Stanford Health Care and professor of medicine at Stanford University. “Very brief exposures to relatively small amounts of virus — you're much less likely to get infected.”