Flu is a contagious illness caused by influenza viruses that infect your nose, throat, and lungs. When you have the flu, you typically feel much worse than with a regular cold: high fever, body aches, exhaustion, coughing, and sometimes nausea. The flu can knock you out for a week or more, keeping you home from school feeling miserable.

The flu spreads easily when infected people cough or sneeze, sending tiny droplets containing the virus into the air. It can sweep through a classroom or family quickly, which is why doctors recommend annual flu shots. These vaccines train your immune system to recognize and fight that year's flu strains.

People sometimes say they have “the flu” when they actually have a stomach bug or a bad cold, but real influenza is more serious. Throughout history, flu pandemics have affected millions. The 1918 Spanish Flu killed more people than World War I. Today's flu vaccines and medical care make it far less dangerous, but the flu still requires rest, fluids, and time to recover. If you get the flu, expect to spend several days in bed feeling truly awful before gradually getting your energy back.