The Flu

Also known as influenza

The flu is a highly contagious illness caused by influenza viruses that infect the respiratory tract (nose, throat, and lungs). Symptoms of the flu, including fever, sore throat, body aches, and cough, tend to come on suddenly. Most people recover from the flu within two weeks. However, young children, the elderly, and those with serious medical conditions can develop complications such as pneumonia. 

Key Terms

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. 

Page Sources
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  1. Tyrrell CS, Allen JLY, Gkrania-Klotsas E. Influenza: epidemiology and hospital management. Medicine. 2021;49(12):797-804. doi:10.1016/j.mpmed.2021.09.015

  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.