The flu (influenza) can cause mild to severe illness. The Flu virus is different from a cold. Influenza usually comes on very suddenly. People who have flu often have some or possibly all of these symptoms:
fever or having chills (not everyone will have a fever)
runny or stuffy nose
cough
sore throat
muscle or body aches
vomiting and diarrhea (this is more common in children than adults)
headaches
fatigue/tiredness