Elevated white blood cell counts, most of which suggest a bacterial infection, are accompanied by elevated neutrophil counts in addition to elevated white blood cells, and such patients require anti-infective treatment. There are many causes of elevated leukocytes alone, and in addition to bacterial infections, patients with viral infections may also have elevated leukocytes. Specific diseases, such as epidemic hemorrhagic fever and infectious mononucleosis, are associated with significantly elevated white blood cell counts in these patients, but with predominantly elevated lymphocytes. Therefore, for determining whether it is a bacterial or viral infection, it is necessary to combine it with other inflammatory indicators, such as C-reactive protein and calcitoninogen. In addition, patients with elevated white blood cells also need to be alerted to hematologic disorders such as leukemia, which requires bone aspiration tests to clarify.