A rare, fast-growing type of leukaemia (blood cancer) in which too many white blood cells called B lymphocytes form in the blood and bone marrow. It may start in the lymph nodes as Burkitt lymphoma and then spread to the blood and bone marrow, or it may start in the blood and bone marrow without involvement of the lymph nodes. Both Burkitt leukaemia and Burkitt lymphoma have been linked to infection with the Epstein-Barr virus.