Flu is a respiratory infection caused by a number of viruses. The viruses pass through the air and enter your body through your nose or mouth. Between 5% and 20% of people in the U.S. get the flu each year. The flu can be serious or even deadly for elderly people, newborn babies, and people with certain chronic illnesses.
Symptoms of the flu come on suddenly and are worse than those of the common cold. They may include
Body or muscle aches
Is it a cold or the flu? Colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. And "stomach flu" isn't really flu at all, but gastroenteritis.
Most people with the flu recover on their own without medical care. People with mild cases of the flu should stay home and avoid contact with others, except to get medical care. If you get the flu, your health care provider may prescribe medicine to help your body fight the infection and lessen symptoms.
The main way to keep from getting the flu is to get a yearly flu vaccine. Good hygiene, including hand washing, can also help.
NIH: National Institute of Allergy and Infectious Diseases