CHAPTER XXIV 



INFLUENZA THE DISEASE AND ITS EPIDEMIOLOGY 



THE INFLUENZA BACILLUS AND OTHER ORGANISMS OF THE 

 HEMOPHILIC GROUP 



The Disease. In no disease is it so difficult to discuss the etio- 

 logical and epidemiological problems as it is in influenza since clinical 

 recognition is not sharp and is fraught with many uncertainties. In 

 time of epidemics there are certain characteristics of onset, sequence 

 of symptoms, and similarity of course which make the diagnosis rela- 

 tively simple. But in interepidemic periods it is not such an easy 

 matter, and the ordinary diagnosis of grippe or influenza is at such 

 times very largely a matter of likelihood, rather than actual recog- 

 nition. 



As will be seen, it is not possible at the present time to base the 

 diagnosis of true influenza upon the isolation of the Pfeiffer bacillus, 

 although this had almost become the habit until the outbreak of the 

 last epidemic. We cannot in this place undertake to describe in detail 

 the clinical manifestations of this disease. For accurate descriptions 

 and historical considerations of both the clinical and epidemiological 

 problems, we may refer the reader to Leichtenstern 's book published 

 in Nothnagel's System, to Thompson's Annals of Influenza (London, 

 1852) and to the more recent treatises in various medical text books. 1 

 We wish to emphasize, however, that in our own experience in the last 

 epidemic, together with a study of the clinical records of previous out- 

 breaks, we have become convinced that pure, uncomplicated influenza 

 is a very mild disease in which respiratory symptoms may be either 

 entirely lacking, or may be extremely mild. This is an important fact 

 to remember in connection with etiological studies, since a very large 

 part of the bacteriological work done on this phase of influenza has 

 unfortunately had to be carried out on cases occurring late in epi- 

 demics when secondary respiratory infection had become almost uni- 

 versal. The pure, uncomplicated cases are found in considerable num- 

 bers only in the early stages of epidemic outbreaks. 



1 See also Zinsser, "Etiology and Epidemiology of Influenza," in the Oxford 

 Medical Papers, 1921. 



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