Sudden, excessive fatigue is one of the very first signs of the flu, according to Healthline. Extreme weakness is another sign of the body preparing to combat the flu rather than a cold.
Body aches and chills are also early symptoms of the flu, as Healthline explains. Aches can happen anywhere in the body, especially the head and legs. Chills may occur at the same time as body aches or within hours of the pain, after which a fever may develop. Coughing, a sore throat and gastrointestinal problems are further indicators of the flu, as are a sore throat and fever.