Cancer is a class of diseases or disorders characterized by uncontrolled division of cells and the ability of these cells to invade other tissues, either by direct growth into adjacent tissue through invasion or by implantation into distant sites by metastasis. This unregulated growth is caused by damage to DNA, resulting in mutations to genes that encode for proteins controlling cell division. Many mutation events may be required to transform a normal cell into a malignant cell. These mutations can be caused by chemicals or physical agents called carcinogens, by close exposure to radioactive materials, or by certain viruses that can insert their DNA into the human genome. Mutations occur spontaneously, or are passed down generations as a result of germ line mutations.
There are many types of cancer. Severity of symptoms depends on the site and character of the malignancy and whether there is metastasis, a process through which cancer cells are transported through the blood or lymphatic system to other parts of the body. A definitive diagnosis usually requires the histologic examination of tissue by a pathologist. This tissue is obtained by biopsy or surgery. Once diagnosed, cancer is usually treated with surgery, chemotherapy or radiotherapy. As research develops, treatments are becoming more specific for the type of cancer pathology. Drugs that target specific cancers already exist for many cancers.
If untreated, cancers may eventually cause illness and death, though this is not always the case. Cancer is a disease that may afflict people at all ages but mainly those of later years. It is one of the leading causes of death in developed countries. Most cancers can be treated and some cured, depending on the specific type, location, and stage. Many forms of cancer are associated with exposure to environmental factors such as tobacco smoke, radiation, alcohol and certain viruses. While some of these can be avoided, there is no known way to entirely avoid the disease, and public health and vaccination programmes are also important on a global scale.