Bone marrow leukemia. Light micrograph of a section through bone marrow from a patient with chronic lymphocytic leukemia (CLL). The bone marrow is seen to be infiltrated by small, abnormal lymphocytes (white blood cells, dark dots). CLL is the most common type of leukemia in adults. It progresses slowly, and the abnormal cells do not severely damage the healthy cell-forming parts of the bone marrow. It may cause no symptoms, or it may cause fever, liver and spleen enlargement and weakening of the immune system. It is incurable, but its course is so prolonged that many patients can live with the disease until they die of other causes.