To infect means to introduce harmful germs into a living thing, causing disease or illness. When bacteria or viruses get inside your body and start multiplying, they infect you. A small cut on your finger can become infected if dirt gets in and bacteria begin to grow, making the area red, swollen, and painful.

Doctors and nurses work hard to prevent infections in hospitals by washing their hands and sterilizing equipment. An infection spreads when germs move from one person to another, which is why you cover your mouth when you cough during cold and flu season.

The word also describes how negative ideas or emotions can spread between people. If one student's bad mood infects the whole classroom, suddenly everyone feels grumpy. When enthusiasm infects a team, everyone catches that excitement and energy. In this sense, both good and bad things can be infectious (capable of spreading), though we usually save the word infect itself for harmful things. You might hear someone say that laughter is infectious, meaning it spreads easily from person to person, but they probably wouldn't say laughter infects people.