Influenza (Flu) What You Need to know What is influenza? Influenza (also called the “flu”) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat and lungs. It can cause mild to severe illness and, at times, can lead to death. The best way to prevent the flu is by getting a flu vaccine each year. Signs and symptoms of flu People who have the flu often have some or all of these signs and symptoms: