The influenza virus, commonly called the flu, is a contagious respiratory infection. Unlike the common cold, the flu causes severe illness and life-threatening complications in many people.Symptoms include fever, Headache, extreme tiredness, dry cough, sore throat pain, runny or stuffy nose, and muscle aches. Children can have additional gastrointestinal symptoms such as nausea, vomiting, and diarrhea, which are uncommon in adults.