What are the causes for low white blood cell count?

Many causes.: Low WBC can be due to nutritional factors, infections (esp. Viral infections), autoimmune diseases, inhereted defects, leukemias and other bone marrow cancers, meylodysplasia, drug reactions, and even unknown causes. The problem with this is infection, but the risk depends on how low the WBC is. Only with very low WBC is there a risk of death.

Multiple causes: Low production due to bone marrow failure, side effects of some drugs, excessive destruction in auto-immune disorders, enlarged spleen from any cause etc. See this site for more info. http://www.mayoclinic.com/health/complete-blood-count/my00476.

Multiple causes: Bone marrow failure, enlarged spleen, auto-immune diseases, some infections and drug effects are some of the causes.

Many: Primary or secondary blood disorder, infection, immunologic diseases, medication, and many other possibilities. Most causes would be treatable. If this has been confirmed then consultation with a hematologist would be appropriate.

Long list: Please consult this site for information on this topic: http://www.mayoclinic.org/symptoms/low-white-blood-cell-count/basics/causes/sym-20050615