When the recent outbreak of influenza appeared, it was assumed by all that it was probably due to the same cause as that of 1889. Health officials, epidemiologists and bacteriologists, already overwhelmed by the demands of the war, were called on to investigate and combat the infection.
The first appearance of the epidemic was in Europe. Bacteriologists, while keeping an open mind, naturally first sought for the influenza bacillus isolated by Pfeiffer from endemic cases of influenza-like infections some two years after the 1889 epidemic.
He had established a relationship between this bacillus and infections of the respiratory tract and had assumed that it was the primary agent in the great pandemic. Investigations since then have thrown considerable doubt on this assumption, and many have now come to consider the bacillus as only one of several varieties of bacteria which have a special selective tendency to attack the mucous membranes