CHAPTER VIII. 



THE INFECTIOUS DISEASES. 

 General Considerations. 



IN the study of the infectious diseases it is especially important to 

 bear in mind that the abnormal processes through which the disturbances 

 incited by micro-organisms are manifested are processes of the body cells 

 and not processes of the micro-organisms. The micro-organisms do 

 indeed incite the train of phenomena by which the disease is mani- 

 fested and the nature or "species" of the micro-organism may largely 

 influence the character of the phenomena, but the stored-up energy 

 which is released in this manifestation is body-cell energy and not 

 that of microbic metabolism. The microbes are excitants of disease, 

 but the disease is a performance of the body cells. If these obvious 

 considerations be held in view, it will be convenient in considering 

 certain of the infectious diseases to use the familiar and much abused 

 term "specific" as indicative of those phases of abnormal body-cell 

 performance which are apt to occur in characteristic ways in response 

 to special forms of microbic stimulus. Thus the poisonous subtances 

 which the tubercle bacillus builds up out of the organic material 

 upon which it feeds are in part such as exert a peculiar influence upon 

 connective-tissue cells, leading to their proliferation and the temporary 

 formation of new tissue the tubercle. This, together with associated 

 action of the same or other metabolic products of the living bacillus, 

 forms a group of lesions and disturbances which is characteristic of the 

 action of the tubercle bacillus in the body. In this sense tuberculosis 

 is a "specific" disease. On the other hand, the poisons eliminated by 

 the tubercle bacillus may incite responses on the part of the body cells 

 which are practically identical with those which many other toxic sub- 

 stances, both of bacterial and of other origin induce, fever, degeneration, 

 etc. These manifestations of the action of the tubercle bacillus upon the 

 living body cells are not "specific." 



In our study of the individual infectious diseases we shall encounter 

 many examples of this variety in the effects which pathogenic bacteria 

 induce the more characteristic on the one hand, and on the other the 

 more general responses which the body cells make to deleterious agents. 



Classification of the Infectious Diseases. It is common to group dis- 

 eases either from the clinical or the morphological or the etiological 

 standpoint. But a complete rational classification of disease is not at 

 present possible, because in very few diseases have we even an approxi- 

 mately complete knowledge of either the symptoms, the excitants, or 

 the morphology of the lesions. 



