Reactive C protein is traditionally considered the most used inflammatory marker. Although procalcitonin has a superior prognostic value, both can create difficulties in differentiating a bacterial infectious process from a viral one. Moreover, no marker is specific for etiology of infection. Complete blood count, investigation of first choice, can provide additional data in order to increase the accuracy of diagnosis. The paper aimed to study the way in which the markers in the complete blood count react during a bacterial or viral infectious process and emphasize their role in confirming diagnosis. The study included two groups, bacterial and viral pneumonia, and studied the following parameters: leukocytes count, percentage of neutrophils, percentage of lymphocytes, neutrophils/lymphocytes ratio, mean platelet volume/platelets ratio, platelets/lymphocytes ratio, mean platelet volume, erythrocyte distribution width, mean corpuscular volume. The study highlighted the usefulness of some of these markers, but only under the conditions of using broader reference values