Information on cancer causation has come from the investigation
of the patterns of cancer in human populations and the
induction of tumors in experimental animals following treatment with cancer-causing agents. The most important human
carcinogens include tobacco, asbestos, aflatoxins, and ultraviolet light. Almost 20% of cancers are associated with chronic infections, the most significant ones being hepatitis viruses (HBV, HCV), papillomaviruses (HPV), and Helicobacter
pylori. There is increasing recognition of the causative role of
lifestyle factors, including diet, physical activity, and alcohol
consumption. Genetic susceptibility may significantly alter the
risk from environmental exposures.