CHAPTER XXYI. 



Infection and immunity— The types of infection ; intimate nature of in- 

 fection—Septicaemia, toxaemia, variations in infectious processes— Immunity, 

 natural and acquired— The bypotlieses that have been advanced in explana- 

 tion of immunity — Ckjnclusions. 



An organism capable of producing disease we call 

 pathogenic or infective, and the process by which it pro- 

 duces disease we know as infection. Diseases, therefore, 

 that depend for their existence upon the presence of 

 bacteria in the tissues are infectious diseases. 



What is the intimate nature of this process we call 

 infection? Is it due to the mechanical presence of 

 living bacteria in the bodv, or does it result from the 

 deposition in the tissues of substances produced by these 

 bacteria that are either locally or generally incompat- 

 ible with life ? Or, is the group of pathological altera- 

 tions and constitutional symptoms seen in these diseases 

 the result of abstraction from the tissues, by the bacteria 

 growing in them, of substances essential to their vitality ? 

 These are some of the more important of the questions 

 that present themselves in the course of analysis of this 

 interesting phenomenon. 



Let us look into several typical infectious diseases, 

 note what we find, and see how far the observations 

 thus made will aid us in formulating an opinion. We 

 begin with a study of those diseases in which there is 

 a general infection — i. e. , in which there is a general dis- 

 tribution of the infective agents throughout the body. 

 This group comprises the " septicaemias," and of them 



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