

INTRODUCTION TO THE STUDY OF INFECTIOUS DISEASES 375 



tions and of those of his clinical predecessors, which have taught 

 him that a certain kind of bacterial invader elicits characteristic 

 reactions on the part of the host. In many cases, however, micro- 

 organisms of entirely different biological classes may elicit clinical 

 pictures of great similarity, because of analogous methods of inva- 

 sion, like degrees of virulence and similar selective distribution. 

 Pneumonias caused by pneumococci and Friedlander bacilli may 

 show very little clinical difference for these reasons, and septicemic 

 invasions of the blood by a variety of microorganisms may not be 

 clinically differentiable. On the other hand, one and the same 

 species of microorganism may cause widely divergent clinical pic- 

 tures under conditions of varying balance between the virulence of 

 the invader and the resistance of the host. A streptococcus of low 

 virulence in a vigorous subject may for instance cause only a local- 

 ized abscess, whereas, if the virulence is enhanced and the subject 

 very susceptible, the localized symptoms may be negligible and a 

 septicemia with secondary localizations and fatal in outcome may 

 result. 



The accurate diagnosis of an infectious disease, therefore, 

 depends first of all upon a clinical understanding of the reactions 

 of the human body with the different microorganisms that can 

 invade it, a knowledge of the manner in which the different forms 

 can enter the body, how they progress and are distributed, where 

 in the body they are apt to accumulate, how great a degree of fluctua- 

 tion in virulence and toxicity can be expected from them, what 

 poisons they produce and what the pharmacological action of these 

 poisons is. While such information may often, and must frequently 

 suffice to make the diagnosis, yet a knowledge of the manner in 

 which blood cultures, urine cultures, stool cultures, throat cultures, 

 etc., can be made, is necessary to affirm the diagnosis in relatively 

 clear cases and to determine it in doubtful cases; and this implies 

 not only a knowledge of the preceding facts, but also both the 

 knowledge and the technical ability supplied by the trained bac- 

 teriologist. 



Moreover, the dependence of clinical understanding of infectious 

 diseases upon bacteriological knowledge is not a bit less intimate 

 than is that of preventive measures or sanitation. 



For purposes of prevention, every infected individual or every 

 carrier of a pathogenic organism may be regarded as the potential 

 source for infection of other human beings. 



