CHAPTER XXVI. 



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

 fectionSepticaemia, toxaemia, variations in infectious processes Immunity, 

 natural and acquired The hypotheses that have been advanced in explana- 

 tion 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 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 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 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. 6., in which there is a general distribution of 

 the infective agents throughout the body. This group 



