Healthy levels of white blood cells in the blood are around 4,500 to 10,000 per microliter, according to National Institutes of Health's MedlinePlus. White blood cells are crucial to prevent infections from occurring in the body, which is why it is important to have the levels monitored. There are many causes of white blood cell levels to be either too high or too low, including a number of diseases.
Low levels of white blood cells in the blood can be caused by severe bacterial infections, cancers, viral illnesses, bone marrow deficiencies and liver and spleen diseases. High levels can be caused by infections, smoking cigarettes, tissue damage, leukemia, inflammatory disease and high amounts of stress.