The flu, or influenza, is a highly contagious respiratory illness caused by influenza viruses. It infects the nose, throat, and sometimes the lungs. Symptoms typically appear suddenly and can range from mild to severe, including fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue. Some people may also experience vomiting and diarrhea, though this is more common in children than adults.