Leukemia is a type of cancer that affects the blood and bone marrow. Certain cells become abnormal and multiply quickly, which can lead to tumors. In 2000, around 256,000 people suffered from leukemia and 209,000 died from it. There are four main types - acute lymphoblastic, chronic lymphocytic, acute myelogenous, and chronic myelogenous - which mostly affect different age groups. Leukemia interferes with normal blood cell production and large numbers of abnormal white blood cells are produced in the bone marrow in children with leukemia. While there is no prevention, leukemia can be treated effectively and even cured in some cases.