Influenza Symptoms Influenza (also known as the flu) is a contagious respiratory illness caused by flu viruses. It can cause mild to severe illness, and at times can lead to death. The flu is different from a cold. The flu usually comes on suddenly. People who have the flu often feel some or all of these symptoms: Fever* or feeling feverish/chills. Cough. Sore throat. Runny or stuffy nose. Muscle or body aches. Headaches.