The flu is famous for its sudden onset. Often, the very first signs are a combination of a high fever (over 100.4°F or 38°C), chills, and significant body aches that appear within a few hours. While a sore throat or cough can be early symptoms, they are usually accompanied by this overwhelming feeling of fatigue and malaise. It's this "hit-by-a-truck" sensation that most clearly distinguishes it from a cold's more gradual progression.