An epidemic is a disease that spreads rapidly among many people in a community at the same time. In the 1980s, the fast-spreading AIDS epidemic transformed life on our planet.
Epidemic can be used figuratively to refer to something that spreads or grows rapidly: an epidemic of laziness has taken over the tenth grade. This word is from French épidémique, ultimately from Greek epidēmia "staying in one place, among the people." The related word pandemic refers to a disease that spreads throughout an entire country or throughout the world.