Major Forms of Cancer - What causes cancer?

In its battle with cancer, medical science devotes constant attention to a search for those factors in our environment that can produce cancer in human beings. They include a large number of chemical agents such as those in tobacco smoke, and including asbestos fibers and other occupational chemical hazards; ionizing radiation such as that from X rays, nuclear bombs, and sunlight; injury or repeated irritation; metal or plastic implants; flaws in the body's immune reaction; genetic mistakes; parasites; and—many scientists believe—viruses.

It is this last factor that is generating perhaps the most interest among medical scientists today. It has been shown that viruses cause a variety of cancers in animals; yet they have never been proved responsible in human cancer, although they have been linked to at least six different ones. Recently, researchers discovered an enzyme in a virus believed to cause cancer and also in the tissues of leukemia patients. This enzyme may be the key to the mechanism by which a virus induces a malignant change in normal cells.

Scientists have also discovered that certain substances in the environment which by themselves may not stimulate the growth of a cancer can be dangerously activated to become carcinogenic by the presence of one or more other substances. Each of these potential cancer-causing agents is called a cocarcinogen . It is possible that some cocarcinogens are present in ordinary fruits and vegetables, in certain food additives, and in such other substances as the synthetic estrogen diethylstilbestrol (DES). For more information on DES, see Ch. 25, .