Immunization is a process that helps your body fight off diseases caused by certain viruses and bacteria, by exposing your body to vaccines, usually by injections (shots). These vaccines contain tiny amounts of material that make your immune system produce certain substances called antibodies. 

These antibodies can attack and destroy viruses and bacteria.

Your immune system stores this information on how to make these antibodies. Later, even many years later, when your body is exposed to that same bacteria or virus, it remembers how to make these antibodies. Your body produces those antibodies again, stopping the virus or bacteria from making you sick.