Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness. Serious outcomes of flu infection can result in hospitalization or death. Some people, such as older people, young children, and people with certain health conditions, are at high risk of serious flu complications. There are two main types of influenza (flu) virus: Types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year.
The best way to prevent flu is by getting vaccinated each year.
Flu and flu vaccine
Learn more about different types of influenza viruses and how these viruses can change over time.
Seasonal Influenza (Flu) frequently asked questions