What is the flu? Influenza (the flu) is an infection of the nose, throat, and lungs caused by influenza viruses. There are many different influenza viruses that are constantly changing. Flu viruses cause illness, hospital stays and deaths in the United States each year. The flu can be very dangerous for children. Each year about 20,000 children younger than 5 years old are hospitalized from flu complications, like pneumonia. See the Flu guide for parents from the CDC