What is the flu and why do I need a flu shot every year?

Updated February 21, 2017 | Factmonster Staff

The ?flu? is a winter illness that is caused by the influenza virus. The influenza virus mutates and changes every year. So every winter the virus that causes the flu is different from the virus of the previous year. Each year a new flu shot is developed to fight against the new virus. And that is why you need a flu shot every year. OUCH!