How Long Are You Contagious With the Flu?

By Sarah Malka, MD
Medically reviewed
November 11, 2021

According to the Center for Disease Control and Prevention (CDC), about 8% of the U.S. population, or roughly 30 million people, get sick with influenza (or the flu) each flu season, which spans fall and winter with peaks typically lasting from December through February.

Though many people only experience mild to moderate symptoms, the flu can develop into a more serious illness, sometimes requiring hospitalization.

Understanding how to spot the symptoms of the flu and for how long you may be contagious can help prevent the chance of spreading the illness to those around you.

In fact, there are several strategies that can help you and your loved ones stay safe and stop the spread of the flu, including limiting person-to-person contact, washing hands thoroughly and frequently, ventilating indoor spaces, and getting the annual flu vaccine.

Read on to learn more about signs you’re contagious with the flu, how to stop the spread, when to see a doctor, and more.

Flu Basics

The flu, also called influenza, is a contagious viral infection that affects all or part of the respiratory system, including the nose, throat, and sometimes the lungs.

There are four strains of the flu virus, including influenza A, B, C, and D. But only two strains, A and B, are responsible for the seasonal flu epidemics that affect the United States each flu season. 

Symptoms

Flu symptoms tend to come on quickly.

Fatigue and fever are usually the first signs, and not everyone with the flu will experience the same symptoms.

Common flu symptoms include:

The flu usually causes mild moderate illness that can be safely managed at home, but some cases can lead to serious illness or death, especially for those with other health conditions. 

Causes

Influenza A and B viruses are responsible for most seasonal flu epidemics in the United States each year.

These flu viruses travel through the air in droplets and are easily spread when an infected person talks, sneezes, or coughs.

These respiratory droplets can land in the mouths or noses of people up to six feet away.

In addition to spreading via respiratory droplets, you can also catch the flu by touching infected surfaces, objects, or by touching an infected person—including shaking hands or wiping a sick child’s nose.

Treatment

For mild or moderate infections, rest, plenty of fluids, and over-the-counter (OTC) medications are often all that’s needed to manage and soothe symptoms.

To treat more severe cases, however, you may want to seek health care from a doctor.

There are several prescription antiviral drugs that are FDA-approved to treat the flu within the first few days of symptoms.

But even when taken after the onset of symptoms, antiviral medications can also help to prevent more serious complications like the need for hospitalization, pneumonia, or death. 

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When You Are Most Contagious

People with the flu are most contagious in the first 3-4 days after their illness begins.

However, some adults can be contagious as early as the day before their symptoms develop.

This is one of the reasons why the flu spreads so easily—people can sometimes spread the illness before they even know they’re sick. 

Healthy adults can remain contagious up to 5-7 days after becoming sick.

Young children and adults with weakened immune systems may be able to spread the virus for an even longer period of time.

Preventing Spread

Simple everyday actions can help prevent the spread of the flu when you’re sick:

  • Limiting person-to-person contact
  • Wearing a mask in public
  • Washing hands thoroughly and frequently
  • Ventilating indoor spaces

However, the most important step in preventing illness is to get the flu vaccine each year.

These vaccines have been shown to reduce flu related illnesses and the risk of severe illness, hospitalization, and death and also prevent spread to others.

It’s important to note that getting vaccinated for the flu once will only protect you for that specific flu season.

Flu vaccines change from year to year depending on which strains of the influenza virus are most prevalent, which is why you should continue to get vaccinated each year for ongoing protection. 

When To Return to School or Work

It can be tempting to return to normal activities as soon as you start to feel better, but it’s important to wait until you can no longer risk spreading the virus to others.

You should wait to return to school or work until 24 hours have passed since your fever has broken.

If the fever only goes away with medication, continue to stay home until your fever has resolved on its own and your symptoms are improving.

When to See a Doctor 

Most cases of the flu are mild and resolve within 1-2 weeks.

However, if you’re immunocompromised, pregnant, or otherwise at risk of severe complications, reach out to your provider if you start to experience any flu symptoms.

Otherwise, healthy adults should reach out to their provider if they don’t feel better within two weeks or if they experience any severe symptoms of the flu, including:

  • Trouble breathing
  • Persistent chest pain and/or pressure 
  • Confusion
  • Inability to wake or stay awake
  • Pale, gray, or blue-colored skin
  • Feeling severely faint or dizzy
  • Vomiting or diarrhea with inability to drink enough fluids to stay hydrated 

How K Health Can Help

Did you know you can get affordable primary care with the K Health app?

Download K to check your symptoms, explore conditions and treatments, and if needed text with a doctor in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

Can you be contagious with the flu if you don’t have a fever?
If 24 hours have passed since your fever has broken (without the help if medication), you can consider yourself no longer contagious and able to resume daily activities. To be safe, continue to wear a mask and wash hands frequently until your symptoms are completely resolved.
Can you be contagious after getting a flu shot?
The flu vaccine is made from an inactivated virus that can’t transmit infection. In other words, the flu vaccine will not infect you with the virus nor make you contagious to others.
How long will the flu last?
Not everyone with the flu will experience the same duration of symptoms. However, most people will recover within one week to two weeks, though a cough can persist 3-4 weeks after the illness.
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Sarah Malka, MD

Dr. Sarah Malka is a board certified emergency medicine physician with K Health. She completed her residency at Harvard Medical School.