There are two main types of influenza (flu) virus: Types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year. Influenza A viruses can be broken down into sub-types depending on the genes that make up the surface proteins. Over the course of a flu season, different types (A & B) and subtypes (influenza A) of influenza circulate and cause illness.

More Information about Flu Viruses

• Types of Influenza Viruses
  Influenza A and B viruses are responsible for seasonal flu epidemics each year.
• How the Flu Virus Can Change
  Influenza viruses can change in two different ways—antigenic drift and antigenic shift.
• Images of Influenza Viruses
  Graphics of generic influenza viruses
• Transmission of Influenza Viruses from Animals to People
  Influenza A viruses also are found in many different animals, including ducks, chickens, pigs, whales, horses and seals.
• 2009 H1N1 Influenza
  The influenza A (H1N1) virus that emerged in 2009 caused the first global influenza pandemic in more than 40 years.

Questions & Answers about Flu Viruses

• Influenza (Flu) Research
  Information about CDC’s past and current research on influenza viruses