Leukemia, or cancer of the blood or bone marrow, occurs in both acute and chronic forms. While the exact causes of the disease are not known, several risk factors have been identified. There are four major types of leukemia: acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphoid leukemia (ALL), and chronic lymphoid leukemia (CLL), and treatments include chemotherapy, radiation, and bone marrow transplants. "Leukemia" discusses the science of leukemia, theories about its causes, the history of the disease, and the current treatments and how they work.