To immunize someone means to protect them from a disease by giving them a vaccine. When you get immunized against measles or chickenpox, a doctor or nurse gives you a shot containing a weakened or inactive form of the germ that causes the disease. Your body learns to recognize and fight that germ, so if you encounter the real thing later, your immune system already knows how to defeat it.

Immunization has saved millions of lives throughout history. Before vaccines existed, diseases like polio left thousands of children paralyzed every year, and smallpox killed entire communities. Today, thanks to immunization programs, many of these diseases have nearly disappeared. When you get immunized, you protect yourself and also help protect people around you who might be too young or too sick to get vaccinated themselves.

The word can also mean to make someone resistant to something non-medical. If you practice public speaking many times, you might become immunized to stage fright. The constant experience builds up your confidence the same way a vaccine builds up your body's defenses.