Influenza (also known as flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat and lungs. Flu is different from a cold, and usually comes on suddenly. Each year flu viruses cause millions of illnesses, hundreds of thousands of hospital stays and thousands or tens of thousands of deaths in the United States. Flu can be very dangerous for children. CDC estimates that between 6,000 and 26,000 children younger than 5 years have been hospitalized each year in the United States because of influenza. The flu vaccine is safe and helps protect children from flu.