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.