CHAPTEE XXVI. 



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

 infection — Septicsemia, toxsemia, variations in infectious processes 

 — Immunity, natural and acquired — The hypotheses that have 

 heen advanced in explanation of immunity — Conclusions. 



An organism capable of producing disease we call 

 pathogenie or infeotive, 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 body, or does it result from the 

 deposition in the tissues of substances produced by these 

 bacteria that are either locally or generally incompa1> 

 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 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, 

 534 



