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Understanding Flu Viruses

Influenza (Flu) Viruses

There are two main types of human flu viruses: types A and B. The flu A and B viruses that routinely spread in people are responsible for seasonal flu epidemics each year.

Flu 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 (flu A) of flu circulate and cause illness.

More Information about Flu Viruses

Types of Flu Viruses
Flu A and B viruses are responsible for seasonal flu epidemics more commonly known as the flu season.
How Flu Viruses Can Change
Flu viruses can change in two different ways—antigenic drift and antigenic shift.
Transmission of Flu Viruses from Animals to People
Flu A viruses also are found in many different animals, including ducks, chickens, pigs, horses, whales, and seals.
- Avian (Bird) Flu: Information on flu viruses in birds
- Canine (Dog) Flu: Information on flu viruses in dogs
- Swine (Pig)/Variant Flu: Information on flu viruses in pigs and human infections with flu viruses from pigs
Pandemic Flu
A flu pandemic is a global outbreak of a new flu A virus. Learn more, including about past flu pandemics.
Images of Flu Viruses
Graphics of generic flu viruses
Human Serology and Flu
CDC conducts human serology work to improve seasonal flu vaccines and prepare against future flu pandemics.
Antigenic Characterization of Flu Viruses
CDC antigenically characterizes circulating flu viruses each year to monitor for changes and to help inform flu vaccine composition recommendations.
Flu Virus Genome Sequencing and Genetic Characterization
CDC studies genetic changes in circulating flu viruses to support public health objectives.
Advanced Molecular Detection (AMD) and Flu
CDC’s Advanced Molecular Detection (AMD) initiative uses new technology to study flu viruses faster and in more detail than ever before.

Types of Influenza

Understanding Flu Viruses