Influenza, commonly called the flu, is a contagious, systemic respiratory illness that can affect both the upper and lower respiratory tracts. It can cause mild to severe illness, and at times can lead to death.
Seasonal flu epidemics affect millions of people in the United States every year. Experts recommend that everyone six months of age and older receive a vaccination, or flu shot, every year.
It's important to note that many people misuse the term "flu" to describe a digestive illness that causes vomiting and diarrhea. This condition is actually called gastroenteritis, and is not caused by the same viruses that cause influenza.
Occasionally, vomiting and diarrhea can accompany illness caused by influenza, especially in young children, but the flu is primarily a respiratory condition, affecting the nose, throat, and lungs.