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H1N1 influenza (Swine flu)

Swine flu; H1N1 type A influenza

The H1N1 virus (swine flu) is an infection of the nose, throat, and lungs. It is caused by the H1N1 influenza virus.

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Causes

Earlier forms of the H1N1 virus were found in pigs (swine). Over time, the virus changed (mutated) and infected humans. H1N1 is a new virus first detected in humans in 2009. It spread quickly around the world.

The H1N1 virus is now considered a regular flu virus. It is one of the three viruses included in the regular (seasonal) flu vaccine.

You cannot get H1N1 flu virus from eating pork or any other food, drinking water, swimming in pools, or using hot tubs or saunas.

Any flu virus can spread from person to person when:

• Someone with the flu coughs or sneezes into air that others breathe in.
• Someone touches a doorknob, desk, computer, or counter with the flu virus on it and then touches their mouth, eyes, or nose.
• Someone touches mucus while taking care of a child or adult who is ill with the flu.

Symptoms, diagnosis, and treatment of H1N1 influenza are similar to that for the flu in general.

Related Information

References

Centers for Disease Control and Prevention website. About swine/variant flu. www.cdc.gov/swine-flu/about/index.html. Updated July 8, 2024. Accessed July 6, 2025.

Treanor JJ. Influenza viruses, including avian influenza and swine influenza. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 165.

Review Date: 4/1/2025  

Reviewed By: Linda J. Vorvick, MD, Clinical Professor Emeritus, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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