Several causes have been identified so far behind the development of Cancers. The causes of cancer can occur due to different factors. The main causes include mutation due to chemical carcinogens; mutation due to ionising radiation; viral or bacterial infection; hormonal imbalances; immune system dysfunction; and heredity. Abnormalities in the genetic material of the transformed cells are the main cause behind the development of most types of cancers. The effects of different substances may cause these abnormalities. The substances like tobacco, radiation, chemicals, or infectious agents, etc. are some of the major causes behind these abnormalities. The cancer-causing substances are also known as carcinogens. Certain environmental factors are also identified as potential causes of cancer.

Mutation due to chemical carcinogens is a major cause behind developing cancers. The substances that cause mutations are known as mutagens, and cancer-causing mutagens are called as carcinogens. Different kinds of substances are linked with the development of different types of cancer. The substance like tobacco smoking is a major cause of cancer and is associated with many forms of cancer. Tobacco smoking causes about 90% of lung cancer. Having prolonged exposure to asbestos fibres can cause mesothelioma or carcinoma.

Though, many mutagens can be called as carcinogens, some carcinogens may not act like mutagens. Alcohol is a chemical carcinogen, but not a mutagen. This type of chemicals can promote cancers, by stimulating the rate of cell division. A faster rate of replication leaves less time for the repairing enzymes to repair the damaged DNA during DNA replication, and thus, increases the likelihood of a mutation. The link between lung cancer and tobacco has been established by several researches. Tobacco use is also linked with cancers in other organs like larynx, head, neck, stomach, bladder, kidney, oesophagus and pancreas. Tobacco is responsible for about one in five of all cancer deaths worldwide. Mutation can be caused by ionising radiation also, and it is a major cause of developing cancer. The main sources of ionising radiation include radon gas. Having prolonged exposure to the ultraviolet radiation from sun is possible to lead to melanoma and other skin malignancies. However, the non-ionising radio frequency radiation from mobile phones and other sources are also identified as probable causes of cancer.

Viral or bacterial infection can cause cancers. A few types of cancers are caused by infection with pathogens. There are also many other cancers that originate from a viral infection. This is mostly found for cancers in animals like birds, however, it is active behind developing cancer in humans, as well. Viruses are responsible for about 15% of human cancers worldwide. In fact, viruses are considered as the second most important risk factor for cancer development in humans.

The viruses like human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, and human T-lymphotropic, are mostly associated with human cancers. Among these, the Hepatitis viruses like hepatitis B and hepatitis C, are capable of inducing a chronic viral infection that leads to liver cancer in several hepatitis B patients and hepatitis C carriers, every year. Apart from viruses, certain bacteria are also associated with the development of different types of cancers. The Helicobacter pylori bacterium is found to be closely associated with gastric cancer. However, though only a few of the cells infected with Helicobacter go on to develop cancer, it is found to be quite common among the causes of cancer.

Another cause behind developing cancer is hormonal imbalances. Some hormones in human body can sometimes act in a similar manner to non-mutagenic carcinogens. In this way, they may stimulate excessive cell growth. The role of hyper-estrogenic states in promoting endometrial cancer can be taken as a perfect example of this. The immune system dysfunction is considered as one of the causes of cancers. Association between HIV and many malignancies like Kaposi`s sarcoma, non-Hodgkin`s lymphoma, has been identified in recent researches. Association between HPV and the malignancies like anal cancer and cervical cancer has also been identified. The increased incidence of malignancies in HIV patients is an indication of the change of immune surveillance, being a potential cause of cancer. Apart from these, some other immune deficiency states like common variable immunodeficiency and IgA deficiency are also considered as potential causes of cancers.

Heredity is also found to be the cause of cancers. There are many recognised syndromes of cancer that have a hereditary component. Some inherited mutations are found to be associated with an elevated risk of cancer. The tumours of various organs in multiple endocrine neoplasia are an example of heredity being a cause of cancers. The Li-Fraumeni syndrome, which is actually different kinds of tumours like osteosarcoma, breast cancer, soft tissue sarcoma, brain tumors, can also be considered as examples of heredity, causing cancers. The familial adenomatous polyposis is identified as an inherited mutation of the APC gene, which leads to early onset of colon carcinoma. When occurs in young children, the retinoblastoma or malignant tumour is also considered to be caused due to a hereditary mutation in the retinoblastoma gene of a human being.

There are also other genetic abnormalities which causes cancers. These abnormalities may be randomly acquired through errors in DNA replication, or may be inherited, as well. Thus the abnormalities remain there in all cells from birth. Generally, complex interactions between the carcinogens and the host`s genome, affect the heritability of cancers. New aspects of the genetics of cancer pathogenesis like DNA methylation, and microRNAs are also recognised as important factors behind developing cancers. Another possible cause behind development of cancers is chronic irritation.