Why do we usually get measles or chicken-pox only once?

Measles - we usually get measles or chicken pox only once because our bodies manufacture special chemical defences called antibodies. These antibodies are selective and are effective only against the particular microbe they have been formulated to fight. After battle is won, the antibodies remain in the bloodstream, ready to repel another attack. The antibodies are large complex protein molecules. One group, the antitoxins, act as antidotes, neutralizing the poisons that the microbes release into the body. The other group, the agglutinins, clump the microbes together so that they fall easy prey to the white blood cells, or leukocytes. These leukocytes develop in the bone marrow and are always present in the blood. Some of them surround the infected area and quarantine it by making a wall with their own bodies. Within the barricade, the rest of them attack the microbes and eat them up. Like tiny amoebas, they crawls about and stretch out foot like projections called pseudopods which they use to engulf bacteria and digest them. This explains their other name phagocyte, or eating cell. Each phagocyte can eat about a dozen bacteria this way a process called phagocytosis.