INTRODUCTION 217 



If slightly susceptible, infection is slight, if highly suscep- 

 tible, the infection is severe. 



Evidently the degree of infection is dependent in large 

 measure on the relation between the virulence of the in- 

 vading organism and the susceptibility of the host. High 

 virulence and great susceptibility mean a severe infection; 

 low virulence and little susceptibility, a slight infection; 

 while high virulence and little susceptibility or low viru- 

 lence and great susceptibility might mean a moderate infec- 

 tion varying in either direction. Other factors influencing 

 the degree of infection are the number of organisms intro- 

 duced, the point where they are introduced and various 

 conditions. These will be discussed in another connection 

 (Chapter XXV). 



The study of pathogenic bacteriology includes the thor- 

 ough study of the individual organisms according to the 

 methods already given (Chapters XVIII-XXI) as an aid to 

 diagnosis and subsequent treatment, bacteriological or other, 

 in a given disease. Of far greater importance than the 

 treatment, which in most infectious diseases is not specific, 

 is the prevention and ultimate eradication of all infectious 

 diseases. To accomplish these objects involves further a 

 study of the conditions under which pathogenic organisms 

 exist outside the body, the paths of entrance into and elimina- 

 tion from the body and those agencies within the body itself 

 which make it less susceptible to infection or overcome the 

 infective agent after its introduction. That condition of the 

 body itself which prevents a;iy manifestation of a virulent 

 pathogenic organism after it has been once introduced is 

 spoken of as immunity in the modern sense. Immunity is thus 

 the opposite of susceptibility and may exist in varying degrees. 



That scientists are and have been for some years in posses- 

 sion of sufiicient knowledge to permit of the prevention and • 

 eradication of most, if not all, of our infectious diseases can 

 scarcely be questioned. The practical application of this 

 knowledge presents many difficulties, the chief of which is 

 the absence of a public sufficiently enlightened to permit the 

 expenditure of the necessary funds. Time and educative 

 effort alone can surmount this difficulty. It will probably 

 be years yet, but it will certainly be accomplished. 



