There are two main types of influenza (flu) virus: Types A and B. Influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year. The emergence of a new influenza virus causing illness in people can result in an influenza pandemic, as occurred in the spring of 2009 when a new influenza A (H1N1) virus emerged to cause illness in people and spread worldwide.
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."