Stem cells in the bone marrow produce blood cells such as red cells, white cells and platelets. In aplastic anemia, stem cells get damaged. As a result, the bone marrow becomes empty (aplastic) or contains few blood cells (hypoplastic). The other main cause can be the immune system attacking the stem cells in the bone marrow. The other factors that can injure bone marrow and affect blood cell production include, radiotherapy and chemotherapy treatments, exposure to toxic chemicals, use of certain drugs, autoimmune disorders, viral infections, pregnancy and other unknown factors.