It started with a cough and stuffy nose. But now you're achy, feverish, and exhausted. You just might have influenza, or the flu. Influenza is a type of virus - a microbe that invades plant and animal cells to survive. The flu virus infects your nose, throat, and lungs. It's also very contagious, which means the virus can travel quickly from person to person in your home, your school, and even around the world!

Image credits: main image, © C. S. Goldsmith and A. Balish, CDC.