Cancer is a group of many related diseases.
Normal body cells grow, divide, and die in an orderly fashion. During the early years of a person's life, normal cells divide more rapidly until the person becomes an adult. After that, normal cells of most tissues divide only to replace worn-out or dying cells and to repair injuries. Cancer cells, however, continue to grow and divide, and can spread to other parts of the body. If cells break away from such a tumor, they can travel through the bloodstream, or the lymph system to other areas of the body.