ABSTRACT
Cancer is a disease characterized by cells which proliferate without control, invading neighboring tissue and metastasizing, or spreading groups of cancerous cells through the circulatory or lymph systems to establish new "invasions"in remote tissues throughout the body. The most broadly held theory is that most, if not all, cancers begin with genetic damage in a single cell. The immune system provides continual surveillance of foreign agents into the body, and appears to recognize the uncontrolled cell proliferation of cancer as a foreign invasion. Cancer develops through a series of stages, generally classed as initiation, promotion, and progression. The role of carcinogens is not clear, but initiation-promotion experiments on mouse skin indicate distinct differences among chemicals in their ability to promote the appearence of benign papillomas and the ability to produce cancers. The multistage theory of cancer has been demonstrated in animal experiments and is consistent with epidemiological evidence.
