In his now classic writings on the origin of cancer, Sir Richard Doll1 proposed 3 potential causative factors underlying tumor development: (1) environmental factors, particularly diet, industrial pollution, and viruses; (2) systemic factors, including breakdowns in immunosurveillance; and (3) genetic factors, such as the degree of susceptibility to cancer. Since Doll’s publications in the late 1970s, the biology underlying these proposed causative factors—which were derived largely from epidemiological studies—has been carefully scrutinized to determine the causes of cancer at the biochemical, cellular, and molecular levels. Because of the intensity of biological research and the rapidity with which fi ndings have become available, we now know that cancer is a disease resulting from the interaction of multiple factors at the cellular, genetic, immunologic, and environmental levels.