The frequent association of Bacillus influenzae with the respiratory discharges and with the pneumonias which accompanied the pandemic of influenza, in 1918, gave an apparently reasonable foundation to the hypothesis that this bacillus was the etiologic agent of the initial disease. This assumption seemed at first the more acceptable in view of Pfeiffer's1 discovery of the influenza bacillus in association with influenza during recurrences of the previous epidemic of 1889-1890, and his pronouncement that it was the cause of the disease.
Bacteriologic and pathologic studies made during and after the last outbreak, however, make it very doubtful that the influenza bacillus was more than a secondary invader associated with an as yet unknown virus of influenza.
It was soon found that while Bacillus influenzae was frequently a contributing cause of the pneumonia present in fatal cases, this bacillus was by no means universally in evidence, and there seemed to be
BRANNAN D, GOODPASTURE EW. THE PATHOLOGY OF PNEUMONIA CAUSED BY BACILLUS INFLUENZAE DURING AN INTEREPIDEMIC PERIOD. Arch Intern Med (Chic). 1924;34(6):739-756. doi:10.1001/archinte.1924.00120060002001