Cancer has multiple causes.
ENVIRONMENTAL FACTORS: Environmental factors are generally thought to be responsible for 80% to 90% of all human cancers. The major environmental factors identified so far include:
TOBACCO: Tobacco in various forms of its usage (e.g., smoking, chewing) is the major environmental cause of cancers of the lung, larynx, mouth, pharynx, esophagus, bladder, pancreas and probably kidney. It has been estimated that, cigarette smoking is responsible for more than one million premature deaths each year.
ALCOHOL: Excessive intake of alcoholic beverage leads to esophageal and liver cancer. Beer consumption may be associated with rectal cancer.
DIETARY FACTORS: Smoked fish is related to stomach cancer, dietary fiber to intestinal cancer, beef consumption to bowel cancer, and a high fat diet to breast cancer. Food additives are also some causative agents.
OCCUPATIONAL EXPOSURES: These include exposure to benzene, arsenic, vinyl chloride, asbestos, polycyclic hydrocarbons etc. (e)
VIRUSES: Hepatitis B and C virus is reported to be related to hepatocellular carcinoma. Human papilloma virus (HPV) is associated with cancer cervix.
HABITS AND LIFE STYLE: Habits and lifestyle of people may be associated with an increased risk of certain cancers. Some examples are the established association between smoking and lung cancer, betel chewing and oral cancer, etc. There are several other environmental factors such as sun light, radiation, air and water pollution, pesticides which can cause cancer.
GENETIC FACTORS: Genetic influences have long been suspected. For example, retinoblastoma occurs in the children of the same parent. The Mongol race is more likely to develop cancer (leukemia) than normal children. However, genetic factors are less conspicuous and more difficult to identify.
by Ben Branklin