Flu (influenza) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It spreads primarily through respiratory droplets when an infected person coughs, sneezes, or talks, and symptoms typically appear suddenly. Common signs include high fever, chills, muscle aches, headache, fatigue, sore throat, cough, and congestion, and the illness can range from mild to severe. Certain groups—such as young children, older adults, pregnant individuals, and those with chronic health conditions—are at higher risk for complications like pneumonia. Most people recover within one to two weeks with rest and supportive care, but antiviral medications may be prescribed in some cases. Annual vaccination, good hygiene, and avoiding close contact with infected individuals are key preventive measures.