Immunization is the process by which an individual's immune system becomes fortified against an agent (known as an immunogen). When their system is exposed to molecules that are foreign to the body, the immune system orchestrates an immune response in an attempt to rid the body of that agent. The immune system can develop a much quicker response, should a subsequent attack occur from the same immunogen. This is called immunological memory. Therefore, by exposing the body to an immunogen in a controlled way, the body can learn to defend itself. This is called active immunization.
The earliest mention of active immunization expermentation was with the work of Brodie and Kolmer in the early 1930's. Jonas Salk furthered the study and implementation of immunization with the development of his polio vaccine in 1953.