The immunological system allows humans to remain healthy in spite of attacks from viruses or other germs. When one is exposed to a virus like that of chicken pox or measles, after a few days of illness the organism develops an immune response. When this works the virus is progressively destroyed and the individual returns to health. The organism keeps a durable trace from these episodes know as immunological memory. This allows for an immediate and efficient response in case of a new exposure to the same virus. We say that the individual is now immune to that virus. That is why we only get chicken pox or measles once in a lifetime.