Cancer is a group of many different diseases that have some important things in common.
Cancer affects our cells, the body's basic unit of life.
To understand cancer, it is helpful to know how normal cells become cancerous. The body is made up of many types of cells. Normally, cells grow, divide, and produce more cells to keep the body healthy and functioning properly. The mass of extra cells forms a growth or tumor. Some types of cells are more prone to abnormal growth than others.