Cancer
Cancer (medicine), any of more than 100 diseases characterized by excessive, uncontrolled growth of abnormal cells , which invade and destroy other tissues . Cancer develops in almost any organ or tissue of the body, but certain types of cancer are more lethal than others. HOW CANCER DEVELOPS A healthy human body is composed of 30 trillion cells, most of which are in constant turnover as cells die and others reproduce to replace them in an orderly fashion. Healthy cells of the skin, hair, lining of the stomach, and blood, for example, regularly reproduce by dividing to form two daughter cells (see Mitosis ). This cell division cycle proceeds under the regulation of the body’s intricately tuned control system. Among other functions, this control system ensures that cells only divide when needed, so that organs and tissues maintain their correct shape and size. Should this system fail, a variety of backup safety mechanisms prevent the cell from dividing uncontrollably. In order for a cel