INTRODUCTION 7 



The methods by which immunity may be acquired 

 through active immunization are as follows, enu- 

 merated in their order of efficiency: 



1. Inoculation with virulent living bacteria, typi- 

 fied by the subcutaneous injection of spirilla of Asiatic 

 cholera. 



2. Inoculation with attenuated or relatively aviru- 

 lent microorganisms. This method is exemplified by 

 vaccination against smallpox and antirabic inocula- 

 tion. 



3. Inoculation with dead bacteria. This embraces 

 the prevalent practice of bacterin as prophylactic 

 (vaccine) therapy, notable examples of which include 

 bacterial inoculations against typhoid fever, plague, 

 tuberculosis, furunculosis, carbunculosis, etc. A 

 higher immunity may be produced by supplementing 

 the dead bacterins with inoculations of attenuated and 

 finally living virulent bacteria. 



4. Inoculation with the excreted bacterial prod- 

 ucts. The injection of the horse with the tetanus 

 toxin for the production of antitoxin is the familiar 

 example of this method. 



5. Inoculation with the disintegrated products of 

 dead bacteria (autolysates). Little of value has at- 

 tended this procedure and for practical purposes it 

 may be disregarded. 



