Data are reported of 582 case records of patients with acute pneumonia of different influenza epidemic periods, results of a study of 175 lethal cases due to acute pneumonia, that complicated influenza in adults as well as experimental studies on reproduction of para-influenzal-staphylococcal, influenzal-Klebsiella and Proteus-influenzal infections. The study allowed to find out the causes of diagnostic difficulties, to establish the features of the course of acute pneumonia in patients with respiratory viral diseases to disclose the mechanisms of development of lung changes and make propositions facilitating early clinical diagnosis on the prehospital period.