text: what is cancer?   cancer is a class of diseases characterized by uncontrolled division of cells and the ability of these cells to invade other tissues, either by direct growth into adjacent tissue (invasion) or by implantation into distant sites (metastasis). this unregulated growth is caused by damage to dna, resulting in mutations to genes that control cell division. several mutations may be required to transform a normal cell into a malignant cell. these mutations are often caused by chemicals or physical agents called carcinogens. some mutations occur spontaneously, or they can be inherited (germ line mutations.)  cancer can cause many different symptoms, depending on the site and character of the malignancy and whether there is metastasis. cancer may be painless. 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 radiation.  if untreated, cancers may eventually cause death. cancer is mainly a disease of later years, and is one of the leading causes of death in developed countries. most cancers can be treated and many cured, especially if treatment begins early. many forms of cancer are associated with exposure to environmental factors, such as tobacco smoking, alcohol, and certain viruses. some of these can be avoidable, and public health and vaccination programmes are important on a global scale.