A vaccine is a medicine that trains your body to fight off a specific disease before you ever catch it. When you get a vaccine, usually as a shot, it introduces a weakened or inactive version of a germ into your body. Your immune system, which is your body's defense team against illness, learns to recognize and remember that particular enemy. If the real, dangerous version of that germ ever tries to invade later, your immune system already knows exactly how to defeat it quickly, often before you feel sick at all.

Vaccines have changed human history dramatically. Before vaccines existed, diseases like polio, smallpox, and measles killed or disabled millions of people, including many children. The development of vaccines eliminated smallpox entirely from Earth and made once-common childhood diseases rare. When most people in a community get vaccinated, even those who can't receive vaccines (like babies or people with certain medical conditions) gain protection because the disease has trouble spreading.

The word comes from the Latin vacca, meaning cow, because the first vaccine protected against smallpox using a related virus found in cows. Today, scientists create vaccines for many diseases, from measles and chickenpox to tetanus and flu. Getting vaccinated protects you and also helps protect others around you.