There are three types of flu viruses: A, B and C. The A and B viruses cause epidemics (widespread outbreaks in a country) of infection in people every year in the United States.
- Type C infections cause mild respiratory illness and are not thought to cause epidemics.
- Type A viruses are divided into subtypes. Subtypes of type A that have been found in people worldwide include H1N1, H1N2, and H3N2 viruses.
- Flu viruses are constantly changing. A global flu pandemic (worldwide outbreak) can happen if three conditions are met:
- A new subtype of type A virus is introduced into the human population.
- The virus causes serious illness in humans.
- The virus can spread easily from person-to-person in a sustained manner.
- The H1N1 Flu met all three conditions and caused a worldwide outbreak. In late spring, the WHO declared that a H1N1 flu pandemic is underway.