CHAPTEE XXVI. 



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

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

 natural and acquired — Tlie 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 

 produces disease we know as infection. Diseases, there- 

 fore, 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 results will 

 aid us in formulating an opinion. We begin with a 

 study of those diseases in which there is a general infec- 

 tion, i. e., in which there is a general distribution of the 

 infective agents throughout the body. This group com- 

 prises the " septicaemias," and of them the disease of 



