The moon in question is composed entirely of a radioactive substance, and showers the earth with strong radiation. It is responsible for a particular kind of cancer that causes the cells to multiply out of control fairly quickly. Over time, the victim will mutate into a shambling monstrosity that is a danger to everything around it and will eventually die. The moon is responsible for a number of other anomalies, such as stillbirths and miscarriages. Even more rarely, pieces of moon rock break off and fall to earth, emitting radiation in the surrounding area and putting people at risk.
Despite this, most of the population are unaffected by the moon, and the majority will not contract this disease. Most areas on the planet are also unaffected by the radiation from the moon. The ozone layer doesn't seem to be a viable reason, since it doesn't explain why a relatively few contract and others do not. Why would this be the case? What prevents the moon from mutating and killing the entire planet as well as the population?