Flu refers to illnesses caused by a number of different influenza viruses. Flu can cause a range of symptoms and effects, from mild to life threatening.
Most healthy people recover from the flu without problems, but certain people are at high risk for serious complications.
In the U.S., epidemiologists at the Centers for Disease Control (CDC) work with states to collect, compile and analyze reports of flu outbreaks.
Flu symptoms may include fever, coughing, sore throat, runny or stuffy nose, headaches, body aches, chills and fatigue.
Annual outbreaks of the seasonal flu usually occur during the late fall through early spring. Most people have natural immunity, and a seasonal flu vaccine is available. In a typical year, approximately 5 to 20 percent of the population gets the seasonal flu. Flu-related deaths range from 3,300 to 48,600 (average 23,600).