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(1918 — 20), were carried oul on a very large scale, have in 

 so far lead to a negative result, that now in 1922 we are 

 very far from being so well acquainted with the virus of 

 influenza as they thought they were 30 years ago. 



As is well known influenza is one of the diseases whicli 

 are most difficult to define from the purely clinical standpoint 

 (as well as from the pathological). Apart from the bacteriolo- 

 gical finding we have only a single reliable sign of true in- 

 fluenza as opposed to other catarrhal infections with similar 

 symptoms, — namely its occurrence in the form of pandemics, 

 which by reason of the great infectivity of the disease, are 

 characterised by a more intense-, rapid, and extensive disse- 

 mination than any other epidemic disease. During the whole 

 of the period intervening between 1892 and 1918 the view 

 has been held that, as regards all the catarrhal infections, which 

 in daily language are called „influenza" (and which will be 

 referred to henceforward as „influenza" in inverted commas) 

 and which it is impossible clinically to sharply define, we 

 ought to consider the bacteriological factor as decisive and 

 call the disease produced by Pfeiffer's bacillus true influenza, 

 while all cases where this organism is not found should not 

 be included under the term influenza although epidemiologi- 

 cally, clinically, and pathologically it is impossible to demon- 

 strate any difference. 



This view rests mainly upon Pfeiffer's work and partly 

 on the succeeding investigations on „influenza" which took place 

 in the next few years the results of which agreed on the 

 whole with those of Pfeiffer. 



If however we had not had this knowledge and not disco- 

 vered the „specific" bacillus until 1898 for example we should 

 hardly have had any ground during the following 20 years 

 for regarding it as the virus of influenza or indeed for spe- 

 cially connecting it with this malady. It must certainly also 

 be stated that the „influenza" bacillus, in contradistinction 

 to the microbes of other infectious diseases, has had but slight 

 importance for the clinicians. It has scarcely been usual for 

 doctors to place much importance on getting their „influenza'" 

 cases verified bacteriologically. We have seen it urged that 

 although the presence of the influenza bacillus might be the 

 criterion whether a given epidemic was influenza, the bacillus 



