An epidemic is when a disease spreads rapidly through many people in a community or region during a particular time. When doctors notice that hundreds or thousands of people are suddenly getting sick with the same illness, they call it an epidemic. The flu might cause an epidemic in a city during winter, or chickenpox might spread through a school in an epidemic that affects dozens of students over several weeks.

For something to be an epidemic, it has to be widespread and spreading faster than usual. A handful of kids catching a cold isn't an epidemic, but if half the school comes down with the same virus in two weeks, that might be.

People also use epidemic metaphorically to describe anything spreading rapidly through a group. A teacher might joke about an epidemic of yawning in class, or a principal might talk about an epidemic of kindness when students start inspiring each other to do good deeds. But the word keeps its serious meaning when describing actual diseases, reminding us of times when illness spread quickly and communities had to work together to stay healthy and help those who got sick.