The aim of this prospective study was to assess the value of procalcitonin (PCT) level and serum concentration of C-reactive protein (CRP) in differentiating viral from bacterial infection in selected groups of patients, including those with acute human granulocytic anaplasmosis (HGA), which is caused by an intracellular bacteria Anaplasma phagocytophilum, and patients with the initial phase of tick-borne encephalitis (TBE), which is a viral disease caused by an RNA virus belonging to the Flavivirus genus of the family Flaviviridae.