Skip Navigation

Flu (Influenza)

Skip Content Marketing
  • Share this:
  • submit to facebook
  • Tweet it
  • submit to reddit
  • submit to StumbleUpon
  • submit to Google +

Visit Flu.gov

See Also

Influenza (Flu) Topic for comprehensive information on 2009 H1N1, seasonal, avian (bird), and pandemic flu.

Related Links

 

Confirmed human cases of avian influenza A (H5N1)

Timeline of human and animal H5N1 infections since 1996 (PDF)

Antigenic Shift

The genetic change that enables a flu strain to jump from one animal species to another, including humans, is called antigenic shift. Antigenic shift can happen in three ways:

Antigenic Shift 1

  • A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig.
  • A person passes a human strain of influenza A to the same chicken or pig.
  • When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain.
  • The new strain can spread from the intermediate host to humans.

Antigenic Shift 2

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans.

Antigenic Shift 3

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans.

The new strain may further evolve to spread from person to person. If so, a flu pandemic could arise.

Illustration demonstrating the genetic change that enables a flu strain to jump from one animal species to another, including humans: antigenic shift.

Credit: This image is in the public domain. Please credit the National Institute of Allergy and Infectious Diseases (NIAID).
Illustrator: Links Studio. 
Download: high resolution version of the Antigenic Shift illustration.

View Antigenic Drift

back to top

H1N1 Flu Info Things You Can Do Plan & Prepare International Info HHS.gov CDC.gov U.S. Info

Last Updated January 14, 2011