88 



epidemic the Si antigen has been conspicuously absent. Hence, 

 its association with the Sheffield and Bristol cases, which 

 appeared so interesting, may be merely accidental, and I am not 

 inclined now to lay stress on this serological characteristic as a 

 possible indicator of special pathogenicity. 



Discussion. 



The question may now be discussed whether the results which 

 I have recorded throw any light on the much debated question 

 whether Pfeiffer's bacillus is the cause of influenza. 



I have already remarked that the presence of influenza bacilli, 

 even in large numbers, in the discharges or local lesions of respira- 

 tory disease is not a sufficient argument on which to establish 

 their primary pathogenic activity, since this correlation may 

 represent a secondary invasion, the consequence of their pre- 

 valence in the normal respiratory mucosa. 



In the case of the meningococcus there is a similar wide 

 distribution in the nasopharynx of the general population, 

 which has been particularly noted during epidemic prevalence of 

 cerebro-spinal fever, while the pneumococcus is at all times a 

 common inhabitant of the upper respiratory tract. 



Yet the pathogenicity of these two micro-organisms is not 

 seriously questioned because, (1) in both cases they are present 

 in disease, accompanied by characteristic lesions, in tissues from 

 which they are normally absent, and (2) in both cases recent 

 investigation has shown that the majority of the strains found in 

 disease fall into a few well-defined serological groups. 



In the case of the influenza bacillus the first condition has 

 not been established and, so far as my investigation goes, there is 

 no evidence of the second being fulfilled; the serological 

 characters of the strains isolated from cases of influenza are 

 almost as diverse as those isolated from other sources, including 

 normal persons. 



There is thus no support afforded by the serological reactions 

 in favour of a primary pathogenic relation existing between 

 Pfeiffer's bacillus and influenza. Can it be said on the other 

 hand that the great diversity in serological type excludes the 

 influenza bacillus from being a primary agent in infection ? 

 Again, I think, the answer must be in the negative, at least until 

 the meaning of the similar diversity among other bacterial species 

 which are certainly concerned in the production of epidemic 

 diseases, has been satisfactorily explained. It is conceivable 

 that this very diversity in the case of the B. influenzae species 

 may indicate an active evolution, aiming finally at the emergence 

 of fixed pathogenic types such as are from time to time evolved 

 among the meningococci. Denial of epidemiological importance 

 to the influenza bacillus appears to me, therefore, as premature 

 as the assertion of its exclusive relation to the production of 

 epidemic influenza. 



