XVIII. THE STUDY OF EXPERIMENTAL 

 INFECTIONS DURING LIFE. 



The possession of pathogenetic properties by an 

 organism under study is indicated by the " infection" 

 of the experimental animal a term which is employed 

 to summarise the condition resulting from the success- 

 ful invasion of the tissues of the experimental animal 

 by the micro-organisms inoculated and by their multi- 

 plication therein. Infection is considered to have 

 taken place : 



1. When the death of the animal is produced as a 

 direct consequence of the inoculation. 



2. When without necessarily producing death the 

 inoculation causes local or general changes of a patho- 

 logical character. 



3. When either with or without death, or local or 

 general changes occurring, certain substances make 

 their appearance in the body fluids, which can be 

 shown (in vitro or in vivo) to exert some profound and 

 specific effect when brought into contact with sub- 

 cultivations of the organism originally inoculated. 



The important factors in the production of infection 

 are: 



A. Seed. Virulence of organism. 



Dose of organism. 



B. Soil. Resistance offered by the cells of the 



experimental animal. 



The first two factors, although variable, are to a 

 certain extent under the control of the experimenter. 

 Thus by suitable means the virulence of an organism 



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