The most common cause of a low white blood cell count in the blood is an infection. In most viral infections, the white blood cell count can be slightly low if it is not too severe, but sometimes the white blood cell count can be normal or even elevated. So a low white blood cell count alone does not clearly distinguish between a bacterial infection and a viral infection. In some bacterial infections, the white blood cell count is mostly high, but if the infection is particularly severe, the white blood cell count can even be low. Therefore, the infection should be seen in the context of the overall condition and should not be judged solely by a low white blood cell count. You may also see splenomegaly, as the spleen destroys cells in the blood, and low white blood cell counts can occur, often accompanied by low platelet counts and, in some cases, anemia. Low white blood cell counts can also occur when some blood disorders cause a decrease in blood cell production. Other medications can also cause this.