The immune system works by recognizing foreign invaders using an army of defense cells called lymphocytes. When a person is exposed to a virus or bacterium that they have never been exposed to before, the immune system has only a few lymphocyte “troops” that recognize the invading germ and so it takes a long time to mount a defense. In the meantime, the germs can multiply and cause whatever symptoms characterize that particular disease. In some cases, a person can die before the immune system has time to mount a response.

Vaccination provides a way to expose the immune system to a germ under controlled conditions so the immune system gets a chance to mount a defense. This involves injecting a vaccine, which is a mixture of substances that includes a small amount of a virus or bacterium that has been weakened or killed and so can’t cause disease. Exposure of the body to the weakened or dead germ under such conditions allows the immune system to greatly expand the number of troop cells that are capable of recognizing and defeating the germ. Then if the body is exposed to the germ again, this time under normal conditions, the immune system can mount a swift and overwhelming defense that stops the germ before it can multiply and cause sickness. In some cases this “immunity” provides by vaccines may last forever. In other cases, regular vaccination or boosters may be required to maintain immunity.