Flu (Influenza) is a viral infection of the respiratory system that attacks your nose, throat and lungs. Up to 20 percent of people in the U.S. get the flu each year. The flu can be serious or even deadly for young children, people 65
and older, those who are pregnant and those with certain medical conditions.
Symptoms of the flu come on suddenly and are worse than those of the common cold.
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.