process by which an individual's immune system becomes fortified against an agent

Immunization is a way of preventing a person from getting a disease. This is done by making the person's body come into contact with a bit of the disease so that the body learns how to fight it. The body's ability to fight off a disease is called its immune system.

A child being immunized against polio.

The material is known as an immunogen. Immunization is the same as inoculation and vaccination because inoculation and vaccination use an infecting agent (something that infects) in the same way that immunization does.

When the human immune system is exposed to an infection once, it can learn how to cope quickly if it is exposed to it again. By exposing an individual to an immunogen in a controlled way, the body will be able to protect itself from that infection later on in life.