What conditions can cause decreased bone marrow?

Many diseases can do: Radiation and chemotherapy are common causes. Cancer can get into the bone marrow and replace it, thus crowding the bone marrow cells and prevent normal function. Sometimes the cause is unknown..It is called aplastic bone marrow( aplastic anemia). Cancer of the blood cells(leukemia)can also suppress normal bone marrow and cause low blood counts or sometime high blood counts(but few normal cells).