Diseases that cause low white blood cell counts include aplastic anemia, human immunodeficiency virus, leukemia and lupus, according to Mayo Clinic. Other causes include rheumatoid arthritis, Crohn's disease and infections such as tuberculosis, reports MedlinePlus.
Viral infections such as tuberculosis temporarily disrupt the function of the bone marrow and reduce the number of white blood cells in the blood, reports Mayo Clinic. Autoimmune disorders such as lupus cause the body to attack and destroy bone marrow cells or white blood cells. Hypersplenism is a condition in which the spleen prematurely destroys blood cells.
Aplastic anemia is a condition in which the body stops producing new blood cells, and this causes a low white blood cells count. Patients may have reduced levels of white blood cells due to congenital diseases that diminish the function of the bone marrow. An example is Kostmann's syndrome, which causes low neutrophil production. Myelokathexis is a congenital disorder in which neutrophils fail to enter the bloodstream.
Cancers such as leukemia damage the bone marrow and lead to low white blood cell counts, notes Mayo Clinic. Myelodysplastic syndromes result in the formation of dysfunctional white blood cells, which causes a low white blood cell count. Individuals may also have fewer white blood cells due to severe infections that use up white blood cells faster than the body's rate of production.