Influenza is an infection of the respiratory system -- that is, the lungs and other internal tissues that allow you to breathe. Influenza infections are caused by viruses. It can produce symptoms similar to the common cold, though they're more often severe.
Better known as the flu, influenza comes in three basic types:
- Seasonal influenza. When most people talk about the flu, they are referring to seasonal influenza. You are most likely to develop seasonal influenza in the fall or winter.
- Pandemic influenza. This is a form of flu that passes easily from person to person and quickly spreads around the world.
- Avian influenza. This type of flu is passed between certain types of birds, including ducks and other aquatic birds. Normally, avian flu cannot be spread directly to humans. However, pigs can acquire avian flu and there have been isolated cases of humans developing this type of influenza.