Leukemia is a cancer of the blood or bone marrow. Bone marrow builds blood cells. Leukemia can occur when there is a problem with the making of blood cells. It usually attacks the leukocytes or white blood cells. Leukemia normally affects white blood cells. Your white blood cells are potent infection fighters — they usually grow and divide in an orderly way, as your body needs them. But in people with leukemia, the bone marrow produces abnormal white blood cells, which don't function properly.