Active immunization is the induction of immunity after exposure to an antigen. Antibodies are created by the recipient and may be stored permanently.
Active immunization can occur naturally when a microbe or other antigen is received by a person who has not yet come into contact with the microbe and has no pre-made antibodies for defense. The immune system will eventually create antibodies for the microbe, but this is a slow process and, if the microbe is deadly, there may not be enough time for the antibodies to being used.
Artificial active immunization is where the microbe is injected into the person before they are able to take it in naturally. The microbe is treated, so that it will not harm the infected person. Depending on the type of disease, this technique also works with dead microbes, parts of the microbe, or treated toxins from the microbe. A common example of this form of active immunization is vaccinations, which have led to several controversies in the past and even present regarding their safety.