An increase in WBCs may occur in many conditions including infection (viral, bacterial, fungal, and parasitic), allergy,
leukemia,
multiple sclerosis, hemorrhage, traumatic tap, encephalitis, and Guillain-Barré syndrome.
The WBC differential helps to distinguish many of these causes. For example, viral infection is usually associated with an increase in lymphocytes, while bacterial and fungal infections are associated with an increase in polymorphonuclear leukocytes (neutrophils).