Aplastic anemia

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Aplastic anemia is a disorder in which the cells of the bone marrow that develop into mature blood cells are damaged, leading to low numbers of red blood cells, white blood cells, and/or platelets.
When the bone marrow cells (stem cells) that develop into mature blood cells and platelets are damaged or suppressed, the bone marrow can shut down. This bone marrow failure is called aplastic anemia. The bone marrow failure leads to too few red blood cells (anemia—see also Overview of Anemia), too few white blood cells (leukopenia), and too few platelets (thrombocytopenia).
The term aplastic anemia is used to refer to anemia that occurs when production of all types of blood cells is suppressed. If production of only red blood cells is suppressed, the disorder is called pure red blood cell aplasia.
When the cause of aplastic anemia cannot be diagnosed (called idiopathic aplastic anemia), the cause is likely an autoimmune disorder, in which the immune system suppresses bone marrow stem cells.
Other causes include

Infection with viruses such as parvovirus, Epstein Barr virus, and cytomegalovirus
Radiation exposure
Toxins (such as benzene)
Chemotherapy drugs and other drugs (such as chloramphenicol )
Pregnancy
Hepatitis