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.
- 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.
- New! Antigenic Characterization of Influenza Viruses
- 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