Definition:
The flu is a respiratory illness caused by the influenza virus. There are many different strains of the influenza virus because it mutates so frequently.
The flu is a contagious respiratory infection that usually lasts between 2 and 7 days.
There is a vaccine available on a yearly basis, but it may not always be effective against that year's flu virus. The flu is generally much more severe than a cold.
Also Known As: Influenza
Examples:
The flu is a serious respiratory illness that kills over 35,000 people in the United States every year.
