In medicine immunization is the process by which an individual is exposed to a material that is designed to prime his or her immune system against that material. The material is known as an immunogen. Immunization is different from inoculation or vaccination in that inoculation and vaccination uses a viable infecting agent (i.e., it can make the individual sick) while immunization does not use a viable agent.

When the human immune system is exposed to a disease once, it can develop the ability to quickly respond to a subsequent infection. Therefore, by exposing an individual to an immunogen in a controlled way, their body will then be able to protect itself from infection later on in life.