How Long Is the Flu Contagious?

Flu is contagious for longer than most people expect, and the window opens before you feel ill. That is a big part of why it moves so quickly through households, classrooms, and offices each winter, often passed along by someone who had no idea they were carrying it.

This article sets out the timing the CDC gives for how long people with flu can spread it, who tends to stay contagious longer, and what current CDC guidance says about going back to work or school. It can help you understand where you are in the illness and decide what to do next, but it cannot tell you whether you have flu. Only a test can confirm that.

How long is the flu contagious?

According to the CDC, flu virus can be detected in most infected people from about one day before symptoms develop until 5 to 7 days after they become sick. People with flu are most contagious during the first three days of their illness.

That gives a rough timeline for a typical healthy adult or older child: about a day of spreading the virus without knowing it, roughly three days of peak contagiousness once symptoms hit, and a tail of several more days when it is still possible to pass it on. The CDC also notes that people with no symptoms at all may be able to spread flu, which is part of why it is so hard to avoid.

Flu is one of the most common everyday illnesses covered in our physical health guide, and most people recover on their own. The contagious period matters less for your own recovery than for the people around you, particularly anyone at higher risk of complications.

How soon after exposure do flu symptoms start?

Flu symptoms typically begin about two days after the virus infects your respiratory tract, according to the CDC, though the range runs from one to four days. This gap between catching the virus and feeling sick is called the incubation period.

Because you can be contagious about a day before symptoms appear, the people you live with may already have been exposed by the time you realize you are sick. If someone in your household comes down with flu, it is reasonable to watch for symptoms in yourself over the next several days.

When flu does arrive, it tends to arrive fast. The CDC describes flu symptoms as usually coming on suddenly: fever or feeling feverish with chills, cough, sore throat, a runny or stuffy nose, muscle or body aches, headache, and fatigue. Some people, more often children, also have vomiting and diarrhea. Not everyone with flu runs a fever.

Who stays contagious longer?

Young children and people with weakened immune systems may be contagious for longer than the typical 5 to 7 days, the CDC says. Their bodies can take longer to clear the virus, so they may keep spreading it after they start to feel better.

A weakened immune system, sometimes described as being immunocompromised, can come from a condition such as HIV or some cancers, or from treatments such as chemotherapy or long-term steroids. CDC guidance on respiratory viruses notes that some people with immunocompromising conditions can shed the virus for a long time.

If you care for a young child with flu, or you have a weakened immune system yourself, ask your doctor how long to keep taking extra precautions rather than relying on a general rule.

When can you go back to work or school after the flu?

Under the CDC's current respiratory virus guidance, you can go back to your normal activities when, for at least 24 hours, your symptoms are getting better overall and you have not had a fever without using fever-reducing medication. Both need to be true. The same guidance covers flu, COVID, RSV, and other common respiratory viruses.

Until then, the CDC advises staying home and away from others, including people you live with who are not sick. Going back is not the end of the story, though. The CDC says you may still be able to spread the virus after you feel better, so it recommends added precautions for the next 5 days:

• Cleaner air: open windows, use an air purifier, or meet people outdoors when you can.
• Good hygiene: wash your hands often, cover coughs and sneezes, and clean surfaces that get touched a lot.
• A well-fitted mask when you are around other people.
• Physical distancing, especially from people at higher risk of severe illness.
• Testing when you will be around other people. For COVID, a test can help show how likely you are to spread the virus.

If your fever comes back or you start to feel worse after returning, the CDC says to stay home again and restart the clock: 24 hours of improving symptoms with no fever, then another 5 days of precautions. You can read the full wording on the CDC page about preventing spread when you are sick. Your school, child care center, or employer may have its own rules, so check those too.

How does the flu spread?

Flu spreads mainly through droplets made when people with flu cough, sneeze, or talk, according to the CDC. Those droplets can land in the mouths or noses of people nearby or be breathed into the lungs.

Less often, you can catch flu by touching a surface or object that has the virus on it and then touching your mouth, nose, or possibly your eyes. That is why handwashing and wiping down shared surfaces still matter during flu season.

The everyday steps the CDC recommends to slow the spread are simple:

• Stay home when you are sick and limit contact with others.
• Cover coughs and sneezes with a tissue, then throw it away.
• Wash your hands often with soap and water, or use an alcohol-based hand sanitizer if soap is not available.
• Avoid touching your eyes, nose, and mouth.
• Bring in outdoor air, purify indoor air, or gather outside.

The CDC recommends a flu vaccine every season for everyone 6 months and older. It calls the yearly vaccine the most important step in lowering your risk of flu, and people who are vaccinated but still get sick tend to have milder illness.

How does flu compare with a cold or COVID?

Flu, colds, and COVID share many symptoms, but their timing differs, and the CDC is clear that you cannot tell flu and COVID apart by symptoms alone. A test is needed to confirm which one you have, and our side-by-side guide to telling a cold, flu, or COVID apart walks through the symptoms in more detail.

Question	Common cold	Flu	COVID-19
Timing after infection	Symptoms are usually worst 2 to 3 days after infection	Symptoms usually start about 2 days after infection (range 1 to 4)	Symptoms usually start 2 to 5 days after infection, and up to 14
Contagious period	Usually no longer contagious after the first week	From about 1 day before symptoms until 5 to 7 days after, longer in some people	From 2 to 3 days before symptoms until about 8 days after they start
Most contagious	First 2 to 3 days	First 3 days of illness	About 1 day before symptoms begin
Prescription antivirals	None; care focuses on easing symptoms	Available; work best started within 2 days of symptoms	Available for higher-risk people with mild to moderate illness, started within 5 to 7 days
Back to normal activities	Same CDC rule as flu	After 24 hours of improving symptoms and no fever without medicine, then 5 days of precautions	Same CDC rule as flu

A cold that drags on with facial pain, pressure, and thick mucus raises a different question: whether it has turned into a sinus infection or is still a cold. That has its own timeline and its own reasons to see a doctor.

Can antiviral medicine help, and who should take it?

Prescription flu antivirals can lessen symptoms and shorten the time you are sick by about a day, according to the CDC, and they may lower the risk of some complications, such as pneumonia. They work best when started within two days after flu symptoms begin.

The CDC recommends early antiviral treatment, as soon as possible, for people with flu or suspected flu who are at higher risk of complications, and for people who are very sick, such as those in the hospital. Most people with mild illness who are not at higher risk do not need them. Antivirals are not sold over the counter, so you need a prescription from a doctor or other health care provider.

Antibiotics do not work on flu, because flu is caused by a virus. Taking antibiotics you do not need can cause harm, including rashes, severe allergic reactions, and resistant infections. Whether or not you take an antiviral, the CDC's rule for returning to normal activities is the same, because it is based on your symptoms and fever.

Who is at higher risk of flu complications?

The CDC lists several groups at higher risk, including:

• Adults 65 and older
• Children younger than 5, and especially those younger than 2
• Pregnant women, including up to 2 weeks after the end of pregnancy
• People with asthma or other chronic lung disease, heart disease, diabetes, kidney or liver disorders, or a history of stroke
• People with a weakened immune system
• People with severe obesity
• Residents of nursing homes and other long-term care facilities

If you are in one of these groups and think you have flu, contact your doctor promptly rather than waiting to see how it goes, because the two-day window when antivirals work best passes quickly.

How can you feel better while you recover?

Most people with flu recover on their own in a few days to less than two weeks, according to the CDC. While you recover, the goals are to rest, stay hydrated, and keep your distance from others as much as you can.

• Rest and fluids: fever and a poor appetite make it easy to get dehydrated, so sip water, broth, or other drinks through the day.
• Fever and aches: over-the-counter pain relievers can ease fever, headache, and body aches. Read the label, and check with a pharmacist if you take other medicines. Do not give aspirin to a child or teenager with flu unless a doctor tells you to.
• A sore throat: warm drinks, honey, and soft or cold foods can make swallowing easier. Our list of foods that soothe a sore throat has more ideas.
• A lingering cough: a cough can hang on after other symptoms fade, and our guide to home remedies for a stubborn cough covers what may help. Never give honey to a baby younger than 1 year.

A cough that keeps going for weeks, or a fever or cough that gets better and then comes back worse, deserves a call to your doctor. The second pattern is on the CDC's list of emergency warning signs, because it can point to a complication such as pneumonia.

When should you call a doctor or 911?

Call your doctor if you are at higher risk of complications and think you have flu, if you are very sick or worried, or if your symptoms are not improving. Get medical care right away, by calling 911 or going to the emergency room, for any of the CDC's emergency warning signs below.

Emergency warning signs in adults:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent dizziness, confusion, or being unable to wake up
• Seizures
• Not urinating
• Severe muscle pain
• Severe weakness or unsteadiness
• Fever or cough that improves but then returns or gets worse
• Worsening of a chronic medical condition

Emergency warning signs in children:

• Fast breathing or trouble breathing
• Bluish lips or face
• Ribs pulling in with each breath
• Chest pain
• Severe muscle pain, such as a child refusing to walk
• Dehydration: no urine for 8 hours, a dry mouth, or no tears when crying
• Not being alert or interacting when awake
• Seizures
• Fever above 104°F that is not controlled by fever-reducing medicine
• Any fever in a baby younger than 12 weeks
• Fever or cough that improves but then returns or gets worse
• Worsening of a chronic medical condition

The CDC notes that these lists are not complete. Contact a doctor about any other symptom that is severe or concerning, and trust your instincts about a child who seems seriously unwell.