Low white blood cell counts are commonly caused by viral infections, conditions that cause depleted bone marrow, severe infections, and certain medications that eradicate white blood cells, according to Mayo Clinic. Specific medical conditions that cause low white cell counts include leukemia, aplastic anemia and lupus.
Additional conditions that cause depleted white blood cells include rheumatoid arthritis, Crohn's disease and tuberculosis, explains MedlinePlus. Low white blood cells are also referred to as neutropenia. Individuals with less than 1,700 white blood cells per liter of blood are at higher risk of contracting illnesses and developing infections. When the count drops below 500 cells per liter of blood, the condition is referred to as severe neutropenia.