A person with leukemia has cancer of the white blood cells. The white blood cells multiply out of control in the bone marrow, resulting in abnormally high numbers of white blood cells in the bloodstream.

Leukemias are classified by the type of cell that is affected. Childhood leukemia usually occurs between the ages of 3 and 7 years. Adults are usually affected by leukemia in their 50s or 60s.