Welcome to the CDC Seasonal Flu website.
Skip directly to the search box, site navigation, or .

  • Contact Us
  • 800–CDC–INFO
  • (800-232-4636)
  • 888–232–6348 (TTY)
  • cdcinfo@cdc.gov

Types of Influenza Viruses

There are three types of influenza viruses: A, B and C. Influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes, Influenza A viruses can be further broken down into different strains. The current subtypes of influenza A viruses found in people are A (H1N1) and A (H3N2).

Influenza B viruses are not divided into subtypes. Influenza B viruses also can be further broken down into different strains.

Influenza A (H1N1), A (H3N2), and influenza B strains are included in each year's influenza vaccine. Getting a flu vaccine can protect against influenza A and B viruses. The flu vaccine does not protect against influenza C viruses.

Related information on Influenza A(H1N2) can be found at:

CDC logo
USA.gov: The U.S. Government's Official Web Portal Department of Health and Human Services