Flu is caused by a virus that is spread from person to person. It causes seasonal epidemics each year.
Influenza (known as the flu) is a common and contagious illness caused by a virus. There are four types of influenza virus, and two of them (types A and B) are responsible for seasonal flu and flu epidemics. Type C only causes mild illness in people and doesn’t cause epidemics. Type D can infect cattle, but doesn’t cause illness in people.