Cancer cells are the basis of disease: they initiate tumor and lead to tumor progression, carrying mutations in oncogenes and tumor-supressor genes that define cancer as a genetic disease. Benign tumors are usually changes with a good prognosis for life. Malignant lesions may be treatable disease if detected at an early stage and, if followed immediately after appropriate therapy. Therapy of malignant tumors often fails to completely cure the disease but can extend life span, usually just a few years. Abnormal cells occasionaly develop in the body of every man and under normal circumstances, the immune system of the body carefully searches for them and destroys them. Their way of life decide whether they are activated and whether our body will be able to defeat them. For cancer to develop, there must be a disruption of the immune system. It is interesting to find that between illness and stress there is a strong connection. Association between stress and disease is so strong, that the level of stress in a person's life, it is possible to predict the occurrence of disease. Emotions can affect the development of disease and emotional responses to stress, which may lead to a tendeny to cancer. This discovery is of a great importance for people suffering from cancer because it shows that the effects of emotional stress can weaken the immune system. So that real prevention against cancer is not early detection, but rather changes in habits and lifestyle so that the disease would not occur.