198 THE INFECTIOUS DISEASES. 



In the infectious diseases as we now define them, the excitant is 

 definite and in many cases known, but a classification based upon the 

 character of the excitants alone would be, as Martins has urged, a classi- 

 fication of the micro-organisms and not a classification of the diseases. 

 If every micro-organism capable of exciting disease always met in the 

 body a similar response, the matter would be comparatively simple. 

 But the fact that the responses of the body cells to bacterial invasion are 

 exceedingly varied, and that dissimilar organisms may evoke similar 

 responses renders a simple etiological classification even of the infections 

 diseases unsatisfactory if not impracticable. Thus it is that it is con- 

 venient to consider the infectious diseases in part together, in part in 

 connection with the special organs in which their more common and 

 characteristic lesions are manifested. Such a classification of the in- 

 fectious diseases as is here made is based in part upon similarity of 

 lesions, in part upon the relationships of the micro-organisms concerned, 

 and may wisely be regarded only as a convenient form of catalogue. 



Groups of Bacterial Disease-Excitants. One of the interesting results 

 of the later studies of bacteria and their associations with the infectious 

 diseases is the discovery that many micro-organisms which have been 

 proved to be excitants of disease in men or in lower animals are closely 

 related to forms which are not pathogenic. So that we now recog- 

 nize many bacterial groups which we are wont to characterize by the 

 name of the pathogenic representative. Thus there are staphylococ- 

 cus and streptococcus groups of closely similar organisms, most of them 

 harmless to man. There is the colon bacillus group, embracing many 

 closely related forms difficult to identify. The tubercle bacillus group, 

 the diphtheria bacillus group, the actinomyces or streptothrix group, are 

 other examples of this relationship. J The more these related forms are 

 studied, the more evident it becomes that in very slight physiological 

 variations may lie the difference between pathogenic and non-pathogenic 

 forms, and that equally slight variations in the susceptibility of the host 

 may be of corresponding significance. 



In the arrangement and associations of the infectious diseases consid- 

 ered in this section, the existence of these bacterial groups will be fre- 

 quently recognized. 



SUPPTJBATIVE AND ALLIED FORMS OF INFLAMMATION. 



\Ve have seen in an earlier part of this book that in various kinds of 

 injury to the living tissues there may be a series of responses on the part 

 of the body cells which constitute or give rise to the phenomena and 

 lesions of inflammation. One of these forms of tissue response to injury 

 is called suppuration or suppurative inflammation. 



We have seen that the characteristic feature of suppurative inflam- 

 mation is the collection at or near the seat of injury of leucocytes, mostly 



1 This grouping of related forms, frequently with special reference to the qualities 

 of particular species as excitants of disease, has been specially worked out by Kruse. 

 See Flugge's " Mikroorganismen, " Bel. ii. 



