Influenza (Flu) Viruses
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.
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.
Antigenic Characterization of Influenza Viruses
“Antigens” are molecular structures on the surface of viruses that are recognized by the immune system and are capable of triggering an immune response (antibody production).
2009 H1N1 Influenza
The influenza A (H1N1) virus that emerged in 2009 caused the first global influenza pandemic in more than 40 years.
Images of Influenza Viruses
Graphics of generic influenza viruses
- Page last reviewed: October 5, 2017
- Page last updated: December 15, 2017
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
- Page maintained by: Office of the Associate Director for Communication, Digital Media Branch, Division of Public Affairs