Influenza (flu) is an infection, caused by a virus, that makes you feel very sick, often with fever, headache, body aches, and coughing.
People often use the term "flu" to describe a cold or a stomach virus. But influenza isn't a stomach problem (although some little children may have vomiting and diarrhea). And it usually feels much worse than a cold.
E. Gregory Thompson, MD - Internal Medicine & Christine Hahn, MD - Epidemiology