Flu (influenza) usually causes sudden fever, chills, body aches, fatigue, headache, cough, sore throat, and a runny or stuffy nose. Most symptoms last about 5-7 days, though tiredness and weakness can persist for 1–2 weeks. Complications are more likely in young children, older adults, or people with chronic health conditions, so it’s best to consult a doctor if symptoms are severe or if you’re in a higher‑risk group.