Increased rates of cancer also have been detected in people exposed to high-strength forms of radiation such as X-rays or radiation emitted from radioisotopes. Because these two types of radiation are stronger than ultraviolet radiation, they can penetrate through clothing and skin and into the body. Therefore, high-strength radiation can cause cancers of internal body tissues. The cancer-causing ability of high-strength radiation has been shown in several instances. Examples include cancer caused by nuclear fallout from atomic explosions and cancers caused by excessive exposure to radioactive chemicals.
Chemicals and radiation that are capable of triggering the development of cancer are called "carcinogens." Carcinogens act through a multistep process that initiates a series of genetic alterations ("mutations") and stimulates cells to divide and grow (proliferate). A prolonged period of time is usually required for these multiple steps. This means there can be a delay of several decades between exposure to a carcinogen and the onset of cancer. For example, a group of young people exposed to carcinogens from smoking cigarettes generally do not develop cancer for twenty to thirty years. This period between exposure and onset of disease is the lag time.
In addition to chemicals and radiation, a few viruses also can trigger the development of cancer. In general, viruses are small infectious agents that cannot reproduce on their own, but instead enter into living cells and cause the infected cell to produce more copies of the virus. Like cells, viruses store their genetic instructions in large molecules called nucleic acids. In the case of cancer viruses, some of the viral genetic information carried in these nucleic acids is inserted into the chromosomes of the infected cell, and this causes the cell to become malignant.