Reticulocytopenia, also known as an "aplastic crisis" or "marrow failure", is the medical term for an abnormal decrease of reticulocytes in the body. Reticulocytes are immature red blood cells. Reticulocytopenia may be a result of viral parvovirus B19 infection, which invades and destroys red blood cell precursors and halts the red cell production. If infection occurs in individuals with sickle cell anemia or spherocytosis, that will lead to incorporation of two anemia-induced mechanisms: decreased red cell production and hemolysis. The result is a severe anemia (aplastic crisis) which may require blood transfusion.