THE CAUSES OF CANCER
The precise etiology of cancer is not known, but it is thought that heredity, viruses, environmental carcinogens, and possibly emotional stress contribute to its development. Cancer is not inherited, but some families appear to have a genetic predisposition for it. When such seems to be the case, environmentalcarcinogens should be carefully avoided and medical checkups made regularly. Environmental carcinogens include radiation (whether from X-rays, sun, or nuclear wastes), certain chemicals ingested in food or water, some chemicals that touch the skin regularly, and certain substances that are breathed in, such as tobacco smoke and asbestos.
Carcinogens are not known to cause cancer from one or even a few expo-sures, but after prolonged exposure. For example, skin cancer does not develop after one sunburn.