Pancytopenia describes a blood count where the number of red blood cells, white blood cells and platelets are all below normal range. It has numerous causes and a full medical history must be included in any differential diagnosis. It can be caused by chemotherapy, particularly those agents which affect bone marrow, aplastic anemia, leukemia, leishmaniasis, severe vitamin B12 deficiency, lupus, HIV, arsenic poisoning and radiation sickness, among other things.