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A contagious respiratory illness caused by flu viruses, outbreaks of which occur annually during "flu season", the winter months the year in the northern and southern hemispheres. The viruses can cause mild to severe illness, and at times can lead to death though most people have some immunity and a vaccine is available. The flu is characterized by sudden onset, high fever, chills, headaches, dry cough, sore throat, runny or stuffy nose and muscle aches. Stomach symptoms such as nausea, vomiting and diarrhea may also occur, but are more common in children than adults.