'Blood cancer' is the umbrella term used for a variety of malignant diseases of the blood-forming (haematopoietic) system. A distinction is made between two kinds, depending on how they originated: leukaemias and lymphomas. Leukaemias begin in the bone marrow, whereas lymphomas start in the lymphatic system. All blood cancers involve the degeneration of certain blood cells, which multiply uncontrollably and suppress the healthy blood-forming system. This prevents the blood from performing vital functions, such as fighting infections, transporting oxygen or stopping bleeding.