484 PATHOGENIC MICROORGANISMS 



characteristic of the basic influenza being the susceptibility which it 

 creates to secondary respiratory infection. The possibilities of error 

 in etiological investigation are, therefore, obvious. 



Etiology of Influenza. In 1892, Pfeiffer described the influenza 

 bacillus which he isolated from the sputum of typical cases but failed 

 to find in normal controls. The organisms were present in large num- 

 bers in the sputum of early cases, and in those with pulmonary com- 

 plications which came to autopsy; if the bacterial contents of the 

 respiratory passages were examined* from the pharynx progressively 

 downward to the lung, the influenza bacilli were found in increasing 

 numbers as the examination proceeded toward the smaller bronchi 

 and bronchioles. Since that time, an enormous amount of investiga- 

 tion has been carried out in regard to the relationship of the influenza 

 bacillus to the disease, but we cannot as yet state definitely whether or 

 not it is the actual cause. During the 1889 epidemic and in the years 

 following, much etiological research was done and Huber, Baiimler, 

 Kretz, Kruse, Pfuhl and many others isolated influenza bacilli with 

 great frequency from all the various lesions associated with the disease. 

 In the interepidemic periods influenza bacilli were found with less and 

 less frequency in respiratory infections, but still were found to be 

 present in a great many individuals even when they did not suffer 

 from clinical influenza. It is difficult to come to any definite con- 

 clusions on the etiological considerations involved in influenza at the 

 present time, and it is quite impossible to detail the enormous amount 

 of work that has been done since 1889 on this problem. There are few 

 organisms that appear in the respiratory tract that have not been 

 thought of by someone as possibly causing influenza, but most observers 

 have confined themselves either to the confirmation or refutation of 

 Pfeiffer 's claim concerning the specific significance of the small Gram- 

 negative bacillus which he discovered. We feel confident that if the 

 disease is a bacterial disease at all, no other bacteria need be seriously 

 considered. 



It seems best to us to summarize as briefly as possible the evidence 

 bearing on the etiological problem in influenza, referring the reader 

 for details to the books mentioned to the numerous articles that have 

 appeared on the subject in the last three or four years, and to our own 

 summary mentioned above, in the Oxford Series. 



In the first place, let us reiterate that correct judgment of etio- 

 logical work in this disease must be based upon the recognition that 



