Influenza, commonly known as the flu, is a contagious respiratory tract infection caused by influenza viruses. Influenza can cause mild to severe illness, and at times can lead to death. Symptoms of influenza generally include fever, cough, sore throat, muscle aches, headache, and/or fatigue.
During most influenza seasons, which typically run from November to April, between 10 percent and 20 percent of the population is infected with influenza viruses. More than 200,000 people are hospitalized with influenza each year in the U.S. Certain groups of people, including older adults, young children, pregnant women, and people with certain health conditions, are at higher risk for serious complications from influenza illness.