Influenza is a contagious viral illness that affects your respiratory system: your nose, throat, and lungs. Most people call it the flu, and it's much more serious than a common cold. When you have influenza, you typically develop a sudden fever, body aches, exhaustion, a cough, and a sore throat. Unlike a cold that builds up gradually, influenza often hits hard and fast, making you feel miserable all at once.

The influenza virus spreads easily when infected people cough or sneeze, sending tiny droplets into the air that others breathe in. It can also spread when you touch a surface contaminated with the virus and then touch your face. This is why influenza often sweeps through schools and communities during winter months, when people spend more time indoors together.

Throughout history, influenza has caused devastating pandemics. The 1918 influenza pandemic killed tens of millions of people worldwide, more than died in World War I. Today we have vaccines that help protect against influenza. Doctors recommend getting a flu shot each year because the virus changes constantly, creating new strains that require updated vaccines.