What causes a low white blood cell count?

A low white blood cell count can be related to a number of causes, including viral infections, cancer and diminished immune system function, according to MayoClinic. Detailed blood screening is the first step necessary to determine the specific cause.

A low white blood cell count is called leukopenia. Leukopenia is best defined as a decrease in the number of white blood cells, called leukocytes, in the blood. The test that determines blood cell count is the white blood cell (WBC) test, which is often included as one part of the complete blood count (CBC) test.

Causes of low WBC include HIV, bone marrow damage or disorders, viral infections, leukemia and other illnesses. When diagnosing low WBC causes, one must take into account other factors, including medical history and current symptoms.