Cancer is a group of diseases which affect the cells of the body.
Normal cells reproduce themselves and divide in two.
When cells begin growing too rapidly, and without order, a tumor is created. Some tumors, however, are malignant tumors that invade surrounding tissue, destroying healthy cells. If unchecked, cells may break off and spread through the bloodstream to other parts of the body and create new tumors. Cancer can be divided into four groups. Leukemias are cancers of the blood-making bone marrow.