This, coupled with the fact that cancer is already quite common, can create an impression of an abnormally high number of cases. The term environment includes not only air, water, and soil, but also the home and workplace. People are exposed to a variety of environmental factors for varying lenghts of time, and these factors interact in ways that are still not fully comprehended. Further, individuals have varying levels of susceptibility to these factors. Heredity Research is just beginning to unravel the puzzle of carcinogenesis, in terms of the roles of both heredity and environmental exposures.
Cancers develop because of genetic mutations.
Some mutations that increase the risk of cancer are present at birth in the genes of all cells in the body. If a defective gene is present in all cells, including reproductive cells, this risk-increasing defect can be passed from generation to generation. However, most changes that lead to cancer occur during one's lifetime in genes within the cells of a particular organ. Familial cancer clusters, or multiple cases among relatives, have been reported for many types of cancer. Because cancer is a common disease, it is not unusual for several cases to occur within a family.