Leukemia is a type of cancer that affects the bodys blood-forming tissues, including the bone marrow and the lymphatic system. It is characterized by the rapid growth of abnormal blood cells, which are usually immature white blood cells. The term leukemia comes from the Greek words for "white" and "blood". Unlike other cancers, leukemia doesnt generally form a mass or tumor that shows up in imaging tests, such as X-rays or CT scans. There are many types of leukemia, and some are more common in children while others are more common in adults. Treatment depends on the type of leukemia and other factors.