Leukemia is a type of cancer that affects blood and bone marrow. It occurs when there is an overproduction of abnormal white blood cells, which can crowd out healthy red blood cells and platelets in the body. There are several types of leukemia, including acute lymphocytic (ALL), chronic lymphocytic (CLL), acute myeloid (AML), chronic myeloid (CML), and juvenile myelomonocytic (JMML). Treatment for leukemia depends on the type, severity, and stage of the disease.