Immunization is the process of protecting your body from dangerous diseases by training your immune system to recognize and fight them. When you get immunized, usually through a vaccine, doctors introduce a weakened or inactive form of a germ into your body. Your immune system learns to recognize this invader and builds defenses against it.

Once you're immunized against something like measles or polio, your body remembers how to fight it off. If you ever encounter the real disease later, your immune system attacks it immediately, often before you even feel sick.

Immunization has saved millions of lives throughout history. Diseases that once killed or disabled countless children, like smallpox and polio, have been nearly eliminated through widespread immunization programs.

The process of immunization is sometimes called vaccination, though technically vaccination refers specifically to getting the vaccine shot or medicine, while immunization describes the broader process of becoming immune. Either way, both words describe one of medicine's greatest achievements: teaching your body to protect itself.