Leukemia Leukemia is a disease that affects blood-forming tissues, mainly bone marrow. Leukemia also affects the lymph glands and spleen. Leukemia causes the body to produce an extreme amount of abnormal white blood cells. This causes infections because the abnormal cells cannot stop infections like the normal cells do. Leukemia also causes anemia. Anemia is a disease in which the body makes less blood cells. This happens because the leukemic cells crowd the system. Leukemia also causes excessive bleeding. This happens because the amount of platelets will decrease and clotting will not occur, Researchers think a change in genetic structure c