Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. People who have 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. fatigue (tiredness) some people may have vomiting and diarrhea, though this is more common in children than adults.