Vaccination is a medical procedure that protects you from dangerous diseases by training your body to fight them off. When you get vaccinated, a doctor or nurse gives you a shot (or sometimes a nasal spray or oral dose) containing a weakened or inactive version of a disease-causing germ, or just a piece of it. Your immune system, which is like your body's security force, learns to recognize this invader and builds defenses against it. Then, if the real disease ever tries to attack you, your body already knows how to defeat it quickly.

Vaccination has saved countless lives throughout history. Before vaccines existed, diseases like polio left thousands of children paralyzed every year, and smallpox killed millions worldwide. Thanks to vaccination campaigns, smallpox has been completely eliminated from Earth, and polio has nearly disappeared.

The word comes from vacca, the Latin word for cow, because the first vaccine protected against smallpox using a related virus found in cows. Today, vaccinations protect against dozens of diseases, from measles and chickenpox to tetanus and flu. When enough people in a community get vaccinated, even those who can't be vaccinated (like babies or people with certain medical conditions) gain protection because the disease can't spread easily. This protective effect is called herd immunity.