Cancer is one of the primary causes of death in developed countries. Basically, cancer is characterized by an uncontrolled division of the body cells as a result of DNA damage. Resulting cells with damaged DNA, called tumor cells, have distinct properties: they have unlimited growth potential, increased cell division rate, altered ability to differentiate, they may invade neighbor tissues or create metastases in distant tissues, and others.
Cell proliferation and cell death is strongly regulated in a healthy organism. Certain mutations of the DNA unbalance these processes. So, in essence, cancer is a malady of genes. Mutations in DNA can be provoked by different physical and chemical factors, called carcinogens. There are several theories about origins of cancer, but none of them has been totally proven and explained yet. Still, there are some clues.
The most accepted theory about origins of cancer is the mixed theory. It states than cancer is a poly-etiological malady, which means that it is provoked by a simultaneous action of different agents-carcinogens. Carcinogens are classified in groups, as they have different origins. Microbial carcinogens are substances produced by microorganisms that may cause cancer. A relevant example is Helicobacter pylori, a bacterium that is incriminated of provoking gastric cancer.
Also there are vegetal carcinogens, such as alkaloids. Certain chemicals may also cause cancer. This group includes substances like benzene, asbestos, vinyl-chloride and many more. Firstly, the idea of carcinogenetic influence of certain chemicals was based on epidemiologic observations. For example, in 1775 a high incidence of scrotal cancer was noticed among men who worked as chimney boys. Another example is that the incidence of mouth and throat cancer increased corresponding to smoking popularity. Scientists have found dozens of carcinogens in cigarette smoke, including benzopyrene, nitrosamines and reactive aldehydes.
Another theory about origins of cancer is viruses. The involvement of viruses in the development of certain types of cancer has become of great interest for scientists lately. The theory of viral origins of cancer found its confirmation in complex researches. Scientists have proven that in all cases of cervical cancer is present a virus, called papillomavirus (HPV). More than 100 viruses are classified in this group, affecting different regions, including genital organs, oral cavity and skin.
Papillomavirus is a sexually transmitted disease, and a sexually active woman has a 75% risk to contact it. 10% of infected women will have precancerous changes in the cervix, and about 1% will develop different types of cervical cancer. Another supposed origin of cancer is radiation, due to its ionization effect. The origins of cancer have not been clarified yet, but the involvement of certain factors in cancer development has been proven. The most important carcinogen factors are bacterial, viral, chemical, vegetal, and radiation.