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Tumors usually occur after 40 years, the frequency of the disease increases with age. However, in some cases, tumors can occur in children, for example teratoid tumors, i.e. tumors from embryonic tissue tumors of the nervous tissue, a kind of kidney tumor - the so-called Wilms tumor, tumor of the lymphatic tissue and various angioma - vascular neoplasms.
On the basis of modern experimental studies can be assumed that cause tumors in children may be influenced blastomogenic agents on the fetus during pregnancy the mother.
All the reasons and mechanisms of tumours is still unclear. The study of etiology and pathogenesis of tumors in many respects facilitated the epidemiology of human cancers and numerous experimental studies on animals.
The term "epidemiology" against tumors and other noncommunicable diseases is understood not in the sense of studying the epidemic, and in the original sense of the word, i.e. the study of common people diseases (in Greek "EPI" above and "demos" - people). A comparative study of diffusion of tumors in different parts of the countries, different Nations and different more limited populations have shown that the frequency of the disease in different tumors can be unequal. A comparison of these facts with the living conditions of the population indicates some of the reasons carcinogenesis (the process of tumor development of normal tissue).
So, for example, skin cancer is found in the South more often than in the North, and the most frequent localization on the face and exposed parts of the body. It makes you think that it is excessive exposure to the sun and, in particular, ultraviolet rays, which is proved by experiments on animals.
The incidence of lung cancer, significantly more frequent in the first half of the 20th century in all economically developed countries, are to be delivered in connection with the increase of pollution of atmospheric air with carcinogens (see), the source of which are the emissions of heating systems and industrial enterprises, especially the exhaust gases of internal combustion engines (cars, aviation). The important role played by the inhalation of tobacco smoke Smoking.
Primary liver cancer is found especially often in some countries of Africa and South-East Asia, depends on malnutrition and, in particular, evidently, from some carcinogenic substances that enter into the food.
About the importance of nutrition in the origin of a cancer of a stomach especially convincingly shows that reduction in the incidence of this cancer, for example, in the USA, taken place over the last 40-50 years, which put in connection with changes in the diet of the population and increasing consumption of vitamins. Observations on the cases of the so-called occupational cancer in humans also shed light on some of the causes tumors.
The study of the incidence of lung cancer among miners, mines, where the Deposit of radioactive species, have shown that malignant tumors in humans can be caused by ionizing radiation, in particular the emanation of radium. About this testify meeting before the cases of x-cancer - medical and technical personnel, as well as the excessively OBRUCHEVSKIJJ patients.
Professional bladder cancer employees of aniline-dye industry clearly indicates that some specific chemical (carcinogenic) substances can actually cause cancer in humans.
A large number of experimental studies on different laboratory animals have shown the possibility to obtain from them a variety of tumors of any organs and tissues with different carcinogenic substances, as well as some radiation. In addition, many experimental works have shown that some tumors in birds and mammals can be caused by viruses. Thus, at present there is no doubt that causes tumors in animals can be some chemicals, radiation and viruses. In humans, the role of viruses in the origin of the tumor is still not proven.