A cancer of blood-forming tissues, hindering the body's ability to fight infection.Leukaemia is cancer of blood-forming tissues, including bone marrow. Many types exist such as acute lymphoblastic leukaemia, acute myeloid leukaemia and chronic lymphocytic leukaemia.Many patients with slow-growing types of leukaemia don't have symptoms. Rapidly growing types of leukaemia may cause symptoms that include fatigue, weight loss, frequent infections and easy bleeding or bruising.Treatment is highly variable. For slow-growing leukaemias, treatment may include monitoring. For aggressive leukaemias, treatment includes chemotherapy that's sometimes followed by radiation and stem-cell transplant.Most blood cancers, also called hematologic cancers, start in the bone marrow, which is where blood is produced. Blood cancers occur when abnormal blood cells start growing out of control, interrupting the function of normal blood cells, which fight off infection and produce new blood cells.