Etiology is the study of causes of a disease. Cancer is neither contagious nor hereditary disease. It is suggested that every living organism has some inactive cancer-causing genes called proto-oncogenes. A number of physical, chemical or biological agents are known to mutate and activate these proto-oncogenes into active and cancer causing oncogenes. Due to altered gene activity, normal control mechanism is lost and the abnormal cell growth and cell division take place. The physical and chemical agents, which induce cancer growth, are called carcinogens.
1. Ionising radiations like X-rays, gamma-rays and particulate radiations from radioactive substances are known to rupture DNA strands and induce mutations to cause cancers e.g., excessive exposure to sunlight may stimulate the development of skin cancer in farmers. Evidence for the carcinogenic effect of X-rays is documented that the incidence of leukemia is about ten times in radiologists than in other physicians. Japanese people exposed to radiations released from World War II nuclear explosions shoved five times the incidence of leukemia than the rest of the population.
2. Physical irritants like certain foods, which cause continued abrasion of the linings of the intestinal tract, are also carcinogenic. Kashmiris use kangri to keep themselves warm but is known to cause abdominal skin cancer due to constant heat exposure.
3. Chemical agents like caffein, polycyclic hydrocarbons, heavy metallic ions etc. are also carcinogenic. Hormones like testosterone and estrogens are known to cause prostate and breast cancer respectively. Chewing of beetles is known to cause mouth cancer. Cigarette and cigar tobacco smoking causes lip, mouth and lung cancers due to presence of a carcinogenic agent, benzpyrene and N-nitroso-dimethylene. An animal protein-rich diet is known to cause cancer of large intestine. Dye workers have a high rate of bladder cancer.
Recently Dr. Leif Busk of Sweden reported that when high carbohydrate foods like potato chips and French fries are heated then a carcinogenic chemical, called acrylamide, is formed.
4. Biological agents. Cervix cancer is caused by viruses. Tumour causing viruses e.g. Epstein-Barr virus, Herpes simplex type-2 virus etc. are called oncoviruses.
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