The causes of cancer are complex and different, but roughly the causes of cancer can be divided into the following six categories: 1. Biological factors, the most important of which is viral infection. Modern researches believe that certain viral infections are closely related to cancer, for example, EBV infection and nasopharyngeal cancer; human papilloma virus infection and cervical cancer, etc. 2. Physical factors. For example, ionizing radiation will lead to many tumors.3. Chemical factors, that is, chemical substances, chemical poisons will lead to certain malignant tumors, for example, xylene, formaldehyde will lead to lung cancer, leukemia, etc.4. Environmental factors. For example, atmospheric pollution, dust pollution in the workplace will induce many cancers.5, psychological factors. Patients with long-term unpleasant mood and depressed emotion are prone to breast cancer, lung cancer and other malignant tumors.6. Hereditary factors. Patients with family history of tumor in their families have increased susceptibility to certain tumors, and the risk of tumor occurrence is significantly higher than that of patients without family history.