How Long Is the Flu Contagious?

Understanding the Incubation Period and Asymptomatic Transmission

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The period of time between exposure to the influenza (flu) virus and the appearance of symptoms—called the incubation period—is around two days. Even so, you can pass the virus to others 24 hours before symptoms begin and for five to seven days after symptoms develop.

This is why it is important to stay home from work or school if you have the flu and to take standard precautions (like washing your hands and covering your mouth while coughing or sneezing) to avoid infecting others.

This article explains how long the flu is contagious, the ways in which it is spread, and what to do if you've been exposed. It also explains how to protect others from influenza if you have it.

Flu Incubation Period
Verywell / Joshua Seong

Contagiousness Before Flu Symptoms

The incubation period for the flu is between one and four days, with an average of two days according to the Centers for Disease Control and Prevention (CDC). This means that if you are exposed to the virus and infected, you will start to experience symptoms (such as fever and body aches) anywhere from 24 hours to four days after exposure.

Once infected, you can be contagious for up to 24 hours prior to the appearance of symptoms. During this asymptomatic (symptom-free) period, you can infect others directly or indirectly without even knowing it.

Risk of Asymptomatic Transmission

In 2021, researchers in South Africa aimed to establish the rate of asymptomatic flu transmission among 1,116 participants in 225 urban and rural households.

The study found that individuals with asymptomatic flu transmitted infection to 29 (6%) of 509 household contacts—meaning that roughly one of every 16 infections was passed from someone with no flu symptoms.

Contagiousness After Flu Symptoms

Once flu symptoms start, adults can pass the virus to others for around seven days. According to the CDC, the virus is most contagious on the third or fourth day. After that, the contagiousness rapidly declines as the immune system gains control over the virus.

In children, the flu can sometimes be contagious for more than seven days. This is because children have less mature immune systems and can take longer to control the virus. The same may apply to older adults and people with compromised immune systems.

As a general rule, the presence of symptoms is a good way to tell if you are still contagious. Until symptoms clear, it is best to consider yourself contagious and take steps to avoid infecting others.

How Does the Flu Spread?

Influenza is a respiratory infection that is mainly spread through respiratory droplets. These are tiny secretions from the mouth or nose that get airborne whenever you cough, sneeze, laugh, or even talk. Once airborne, the virus-filled droplets can travel as far as 6 feet and enter the mouth or nose of someone else.

You can also get influenza through fomite transmission. This happens when respiratory secretions land on surfaces or objects that are later touched by someone else. The virus can then be passed when the person brings their hand to their nose or mouth.

How Long Does Flu Live Outside the Body?

Research suggests that flu viruses can remain infectious outside of the body for several hours, They tend to survive longer on hard surfaces like stainless steel or plastic than on soft, porous surfaces like fabric.

During the cold winter months, the flu spreads rapidly. Contrary to popular belief, it is not due to cold weather. Cold temperatures simply allow the virus to survive longer. Winter winds can also transport the droplets far longer than six feet.

What to Do if You're Sick With Flu

If you have the flu, you should stay home at least 24 hours after your fever has gone without the use of fever-reducing medicines like Tylenol (acetaminophen).

If you don't have a fever but still have symptoms, wait until your symptoms improve and it has been at least four to five days since the start of your illness.

You get over the flu faster with the following steps:

  • Get plenty of rest: Doing too much will not only wear you out but also impair your already-weakened immune system.
  • Drink plenty of fluids: These include water, warm tea, fruit juices, sports drinks, and broths.
  • Treat body aches and fever: This not only includes Tylenol but also nonsteroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen).
  • Ease your cough: An over-the-counter (OTC) expectorant can break up phlegm so you can cough it up easier. Sucking on a cough lozenge or taking a cough suppressant can also help.
  • Take a flu antiviral: When started within one to day days of symptoms, flu antivirals like Tamiflu (oseltamivir), Relenza (zanamivir), Rapivab (Peravir), and Xofluza (baloxavir marboxil) can lessen symptoms and shorten the time you are sick by one or two days.

Warning to Parents

Never give aspirin to children or teens with fever as it can cause a potentially deadly reaction called Reye's syndrome. Children under 4 years should not be given cough or cold medicines as they can cause serious and potentially life-threatening side effects.

Preventing Flu in Others

Flu is something that most people get at one time or another. If it happens, you need to take care of yourself while reducing the risk of infection in others.

The CDC recommends the following:

  • Avoid close contact with others: Isolate yourself from others in your household. If you have to go out, wear a face mask.
  • Cover your mouth and nose: Use tissues when coughing or sneezing, and dispose of them immediately rather than letting them pile up on a counter.
  • Wash your hands frequently: If soap and water are not available, use an alcohol-based hand rub.
  • Avoid touching your mouth, nose, or eyes: If you do, you can contaminate surfaces or objects that other people touch.

Flu Vaccination

The best way to avoid passing the flu is to avoid getting it in the first place. The annual flu vaccine is recommended for everyone over 6 months of age, ideally in September or October, to ensure better protection throughout the flu season.

Summary

Once exposed to the influenza virus, it can take anywhere from one to four days for symptoms to appear and five to seven days more for symptoms to clear. You can be contagious up to 24 hours before symptoms stop and continue to be contagious until your symptoms improve and you no longer have a fever. Peak contagiousness occurs on day three or four of your illness.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  7. National Organization for Rare Disorders. Reye syndrome.

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  9. Centers for Disease Control and Prevention. Healthy habits to help protect against flu.

  10. Centers for Disease Control and Prevention. Seasonal flu vaccines.

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.