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The Influenza (Flu) Viruses
There are two main types of influenza (flu) virus: Types A and B. Influenza A and B viruses are responsible for seasonal flu epidemics each year. Influenza A viruses can be broken down into sub-types. Over the course of a flu season, different types (A & B) and subtypes of influenza A viruses can circulate and cause illness. In addition, influenza viruses are constantly changing through a process called "antigenic drift."