To vaccinate means to give someone a vaccine, which is a medical treatment that helps the body learn to fight off specific diseases before the person ever gets sick. When a doctor vaccinates you, they're usually giving you a shot that contains a harmless version of a germ or just a tiny piece of it. Your immune system (your body's defense system) practices fighting this harmless version, so if the real, dangerous germ ever shows up, your body already knows exactly how to defeat it.

Think of it like a fire drill at school. During a drill, there's no real emergency, but you practice evacuating the building so you'll know exactly what to do if there ever is a real fire. Vaccination works the same way: it trains your body to respond quickly and effectively to diseases like measles, polio, or chickenpox.

Vaccines have prevented millions of deaths throughout history. Diseases that once killed or disabled countless children are now rare in places where vaccination is common. Polio, for example, once paralyzed thousands of American children every year, but thanks to widespread vaccination programs, it was eliminated from the United States in 1979.

A person who has been vaccinated is described as vaccinated or immunized.