How Long Is the Flu Contagious?

Feeling sick? Think it’s influenza? What to expect so you can protect others and recover quickly

Kristen Fischer,

AARP

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Published September 10, 2025

Kyle Hilton

When it comes to the flu, timing is everything. You can be contagious before symptoms even appear and unknowingly spread the virus to others. Determining when you’re no longer infectious is also important so you can safely return to your normal routine.

A flu shot can help lower your odds of coming down with the flu in the first place, and public health experts recommend that almost everyone roll up their sleeves for one, ideally in September or October. That said, no vaccine is 100 percent effective, so in the event you do get sick, here’s what you need to know about how long you’ll be contagious.

When can you detect the flu?

It can be difficult to figure out if you have the flu in the first place. This is because flu symptoms can mimic other respiratory illnesses like COVID-19 and RSV (respiratory syncytial virus).

Flu timeline

Within one day after exposure to the virus: Flu can be detected.
One to two days (or up to four days) after exposure: Flu symptoms start.
One day before showing symptoms: You can become contagious.
One to four days after symptoms show: You’re most contagious.
Up to five to seven days after you get sick: Flu can be detected.
Twenty-four hours without a fever: You’re probably not contagious anymore.

What’s more, flu symptoms can vary with age. For example, older adults may experience malaise (a general feeling of discomfort) instead of a high fever. In fact, average normal body temperatures in older adults are slightly lower than their younger peers, according to the National Institute on Aging, so when older individuals do get sick, their fevers tend to be lower.

Stomach pain, diarrhea and nausea from an influenza infection can be more common in older adults, while nasal congestion, sore throat and runny nose tend to be less so. Still, if you experience any of these symptoms — along with classic flu signs like fever, chills or body aches — head to your doctor’s office or urgent care for a flu test. (You can also find over-the-counter tests at pharmacies and retailers that simultaneously test for flu and COVID-19.)

The Centers for Disease Control and Prevention (CDC) recommends testing for flu as soon as you start to feel ill, ideally within three or four days of symptom onset.

Flu symptoms typically show up within two days of infection, but can develop anywhere from one to four days after infection. If you are infected with influenza, it can be detected one day before symptoms emerge, and up to five to seven days after you start to feel sick.

While it’s possible to spread the flu to others before you start to experience symptoms, you’re more contagious once symptoms appear, says Adolfo Garcia-Sastre, a microbiology professor and director of the Global Health and Emerging Pathogens Institute of Icahn School of Medicine at Mount Sinai in New York.

How long are you contagious?

Once you confirm a flu diagnosis, knowing how long you’re contagious can protect others and give you an idea of when you can return to normal activities.

The first three to four days of illness are when you’re most contagious, public health experts say. That said, you can spread the virus for much longer. The average length of contagiousness is five to seven days, Garcia-Sastre says. “But this can be longer, especially for people with weakened immune systems,” he notes. The CDC says some people may harbor the flu virus and not develop symptoms but still pass it to others.

You can infect others directly or indirectly during your contagious period. Direct contact occurs if you sneeze, cough or talk and droplets you produce land in the mouths or noses of other people nearby. You can also spread the flu indirectly, say, if you touch or cough on an object that someone else comes into contact with.

To avoid spreading the flu, try to avoid close contact when you’re not feeling well and for a few days after, especially in closed environments with poor ventilation, Garcia-Sastre recommends.

“Keeping a distance and wearing a mask can help protect others from contracting the flu if you are sick,” says Albert Rizzo, M.D., chief medical officer of the American Lung Association.

Why is the flu dangerous in older adults?

Older adults are especially prone to complications from respiratory infections like influenza because immune function declines with age. What’s more, chronic conditions that become more common with age, such as diabetes and heart disease, can further exacerbate symptoms in older adults with the flu.

As you age, it can take longer for your body to clear the virus from your lungs, which can prolong inflammation and, in some cases, trigger a heart attack or stroke, explains Laura Haynes, a professor of immunology at the UConn Center on Aging. Flu-related inflammation can weaken muscles, impair mobility and affect the brain. If you’re weak or frail, getting sick with something like the flu can impair your resiliency, too, Haynes adds. Generally, people who don’t exercise, don’t sleep well or don’t consume enough protein may have a harder time recovering from an illness, says Haynes.

How can you treat the flu?

Taking an antiviral medication such as Tamiflu (oseltamivir phosphate), Relenza (zanamivir), Rapivab (peramivir) or Xofluza (baloxavir marboxil) may shorten the amount of time you’re sick and the period of time when you’re contagious, Garcia-Sastre says. These medications are often given within a day or two after symptoms start. “But they need to be taken early during the infection, at the first [sign] of symptoms,” he says.

Once treated, you may be wondering when it’s OK to return to work or see friends. Wait until you’ve been fever-free for at least 24 hours and your symptoms are improving overall, the CDC advises. (If your fever has broken because you’re taking a fever-reducing medication, that doesn’t count.)

“Unfortunately, people with weakened immune systems may be contagious much longer, sometimes for weeks, because their immune systems cannot control the virus well,” says Robert H. Hopkins Jr., M.D., medical director of the National Foundation for Infectious Diseases.

The best way to tell if you’re over the flu is with a test, Garcia-Sastre notes. And it’s important to try to take it easy until you are feeling better physically, Haynes says. The infection can cause dizziness and muscle weakness, which can increase your risk of falling, she explains.

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Kristen Fischer is a contributing writer at AARP based at the Jersey Shore. She has also written for Health, WebMD, and McKnights and is the author of Zoo Zen: A Yoga Story for Kids.