Everything to Know About the Flu, Including How Long It's Actually Contagious

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Photo credit: Mumemories - Getty Images
Photo credit: Mumemories - Getty Images

After being practically non-existent last year, the flu is slowly coming back across the country. The University of Michigan is currently grappling with a flu outbreak and cases of the virus have starting creeping up nationwide.

Rochelle Walensky, M.D., M.P.H., director of the Centers for Disease Control and Prevention (CDC) warned during a press briefing last month that this year’s flu season could be a doozy. "Last year there were very few flu cases, largely because of masking and physical distancing and other prevention measures put in place for the COVID-19 pandemic,” she said. Between October 3, 2020 and July 24, 2021, the CDC saw just 2,136 positive flu tests out of 1.3 million specimens tested by clinical labs, according to data published in JAMA. There were 736 deaths last season from the flu and, by comparison, the 2019–2020 flu season saw an estimated 35 million flu-related illnesses and 20,000 flu-related deaths, per the CDC.

“With modest flu virus activity since March of 2020, CDC's flu experts are concerned that reduced population-level immunity to the seasonal flu could place us at risk for a potentially severe flu season this year,” Dr. Walensky said.

Given that the past two years have made everyone stop and think about how we catch and pass on illnesses, it’s only natural to wonder how long the flu is contagious—especially if you’re unlucky enough to get it. Curious about when you can safely be around others after having the flu? Here’s what you need to know.

What is the flu, again?

The flu (aka influenza) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs, according to the CDC. There are two main types of flu viruses—types A and B—and they usually circulate during flu season every year.

The flu can cause the following symptoms, per the CDC:

  • fever or feeling feverish/chills

  • cough

  • sore throat

  • runny or stuffy nose

  • muscle or body aches

  • headaches

  • fatigue

  • Vomiting and diarrhea (this is more common in children than adults)

How does the flu spread?

The flu is thought to mainly spread by droplets that are created when an infected person coughs, sneezes, or talks. Those droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into their lungs, the CDC explains. Less commonly, a person could get the flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose, or eyes.

How long is the flu contagious?

There’s a spectrum here. People with the flu are most contagious in the three to four days after they develop symptoms, the CDC says. Most people can infect other people up to a day before their symptoms start and up to five to seven days after they show signs of the virus. Kids and people with weakened immune systems may be able to pass on the virus for more than seven days.

But there are a few variables that come into play. “Contagiousness for an infectious disease is not an on and off switch,” says infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. “But it’s important to remember that, as you get further out from when your symptoms started, you could still be contagious.”

Things like taking oseltamivir (Tamiflu) and being vaccinated against the flu should also shorten the amount of time you’re sick—and infectious, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. Still, he says, “there’s not a lot of literature on this. I like to consider people infectious up to seven days, just to be safe.”

As a whole, “people are generally most contagious one to four days after they fall ill,” says David Cennimo, M.D., assistant professor of medicine- infectious disease at Rutgers New Jersey Medical School. “This time of maximal infectivity generally coincides with the time of the worst symptoms [and] most household infections happen right away,” he adds.

If you had the flu and you can swing it, it’s a good idea to try to stay home for a week to avoid infecting others, Dr. Cennimo says. But, if you can’t, Dr. Adalja says you should be OK to venture out after you’ve been fever-free without the use of fever-reducing medication for at least 24 hours.

If it’s within seven days since you were diagnosed and you’re out and about, Dr. Russo recommends wearing a mask to help lower the risk you’ll spread the virus to others. “That’s particularly true if you’re interacting with vulnerable people,” like the elderly, very young, or pregnant people, he says.

And, if you’re feeling better but aren’t sure if you’re OK to get out again, talk to your doctor. They should be able to offer up personalized guidance.

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