The problem of cancer is a challenge to the scientists of the world. It is still wrapped in mysteries about which there exist only the foggiest notions.
Physicians have a considerable fund of knowledge in regard to the agents which can bring about the cellular changes known as cancerous. These inciting factors may be classified as physical, chemical or biologic in nature. The physical agent's include trauma, heat, cold, ultraviolet light, roentgen rays and radium; the chemical agents comprise inorganic as well as the more complicated synthetic organic compounds, internal secretions and vitamins; the biologic agents are the helminths, bacteria and viruses. The evidence that these agents may initiate the growth of cancer has been gathered from human cancers which arise naturally from exposure to them; from occupational and industrial cancers, and from the actual production of cancers in experimental animals by the use of these materials.1
MORTON JJ. THE CANCER PROBLEM. JAMA. 1947;135(15):957-963. doi:10.1001/jama.1947.02890150001001