A group of diseases characterized by uncontrollable cell growth.

Cancer is a family of diseases in which cells replicate at an extremely rapid pace. A cancerous, or malignant, tumor begins its growth at a primary site, damaging surrounding tissue and interfering with normal organ function. Eventually, some of the cancerous cells may migrate from the primary tumor to other parts of the body-via the bloodstream and form secondary tumors. The specific causes of most forms of cancer are unknown, but researchers have uncovered many probable factors that contribute to the development of the disease, including certain chemicals and forms of radiation, cigarette smoking, diet, and, in some cases, genetic predisposition. Although most cancers occur in adults, cancer is still responsible for more deaths in children than any other disease in the United States, where it is the second leading cause of death—after accidents—in children under the...