How come most of us get the "childhood diseases" such as measles and chicken box only once?

Lifelong immunity: Some of the viral illnesses will confer lifelong immunity to those with a normal immune system most bacterial illnesses do not.That is just the way the system works.

Immunizations.....: Have dramatically reduced the incidence of many childhood afflictions, some that were life-threatening or associated with significant morbidity. They are among the great success stories of modern medicine. Proof of this is represented by the difficulty young physicians have diagnosing illnesses that were once common. Notably, we are now seeing these illnesses develop in those with waning immunity.