The Flu

Also known as influenza

The flu is a highly contagious common illness caused by the influenza virus. Symptoms include fever, headache, body aches, runny nose, sore throat, cough, and fatigue that come on quickly. While most healthy people recover from the flu in about a week, children, the elderly, and people with weakened immune systems or chronic medical conditions are at greater risk of serious complications, including pneumonia and even death. 

Two types of influenza viruses cause illness in humans: types A and B. Each type has many strains that mutate often, which is why people continue to come down with the flu year after year—and why flu shots only provide protection for one flu season. You can get the flu at any time of the year, but in the United States, flu season peaks between December and March. 

Frequently Asked Questions

  • How long does the flu last?

    Symptoms of the flu tend to come on suddenly over the course of a few hours and commonly last for four to five days. However, some people may recover from the flu in as few as two days or as many as seven or more days.

  • How long is the flu contagious?

    Influenza is contagious from before you even start to feel symptoms and continues for several days. On average, people are contagious 24 hours before symptoms appear for up to about five days after becoming ill.

  • What are the symptoms of the flu?

    Symptoms of influenza include fever, chills, headache, body aches, runny nose, sore throat, cough, and fatigue. Symptoms tend to come on quickly over the course of a few hours.

  • When is flu season?

    You can catch the influenza at any time of year, but it is most prominent in the fall and winter in the United States. Flu season typically spans from October through April with peak cases between December and March, although flu activity can last as late as May.

  • Can you have the flu without a fever?

    Fever is a common symptom with influenza. Part of the body’s defenses against the virus, an increased body temperature helps to prevent the virus from replicating. However, not everyone will experience a fever when they have the flu. 

  • How is the flu spread?

    Influenza is passed from person to person through infected respiratory droplets shared via sneezing, coughing, talking, or blowing one’s nose. It can also be transmitted via contact with contaminated surfaces. The best way to prevent contracting the flu is to avoid close contact with people who are sick, wash your hands frequently, and get an annual flu shot. 

Key Terms

Page Sources
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  1. Centers for Disease Control and Prevention. About flu. Updated September 5, 2019.

  2. Centers for Disease Control and Prevention. Key facts about influenza (flu). Updated September 13, 2019.

Additional Reading
  • Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020. GeneReviews Glossary.

  • Centers for Disease Control and Prevention. How flu spreads. Updated August 27, 2018.

  • Centers for Disease Control and Prevention. The flu season. Updated July 12, 2018.

  • Centers for Disease Control and Prevention. Types of influenza viruses. Updated November 18, 2019.

  • Centers for Disease Control and Prevention. Flu symptoms & complications. Updated August 31, 2020.

  • Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016;355:i6258. Published 2016. doi:10.1136/bmj.i6258

  • U.S. National Library of Medicine. Vaccines (immunizations). MedlinePlus. Updated October 8, 2020.

  • U.S. National Library of Medicine. Viral infections. MedlinePlus. Updated October 19, 2020.