CHAPTER XXV. 



Infection and immunity The types of infection ; intimate nature of 

 infection Septicaemia, toxaemia, variations in infectious processes 

 Immunity, natural and acquired, active and passive The 

 hypotheses that have been advanced in explanation of immunity 

 Conclusions. 



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 mechanism of this process we call infec- 

 tion ? Is it due to the mechanical presence of living 

 bacteria in the body, or does it result from the deposition 

 in the tissues of substances produced by these bacteria 

 that are either locally or generally incompatible with 

 life ? Or, is the group of pathological alterations and 

 constitutional symptoms seen in these diseases the result 

 of abstraction from the tissues, by the bacteria growing 

 in them, of substances essential to the vitality of both 

 bacteria and tissues ? These are some of the more 

 important 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 to what extent the observa- 

 tions 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. 



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