Leukemia (Leukaemia) is a cancer of the blood cells. It is the most common type of blood cancer and affects 10 times as many adults as children. Most people diagnosed with leukemia are over 50 years old. No one knows why some people develop leukemia and others do not. However, scientists have identified some risk factors for the disease. Most people who have known risk factors do not get leukemia, while many who do get the disease have none of these risk factors. During the early stages of leukemia, there may be no symptoms. Many of the symptoms of leukemia don't become apparent until a large number of normal blood cells are crowded out by leukemia cells.