After organisms are exposed to disease the immune system has a memory system.

This memory system is the original antibody that the body made in order to attack the foreign invaders.

After the first exposure, primary immune response, the immune system lays out a memory, and when re-exposed the body will quickly make antigens so that the organism will not get the disease.

Vaccinations are the use of dead or weakened microbes or parts of them that will stimulate the initial immune response.

Using this lesser form of the disease allows the organism to not become as sick as they would if they caught the disease. The body will then have the antigens to fight off the disease if it tries to enter the body again.