508 THE BACTERIOPHAGE AND ITS BEHAVIOR 



community of individuals, the struggle which occurs within the single 

 infected individual between the bacterium and the protobe. 



The bacteriophage protobe is transmissible from one individual to 

 another just as is the bacterium itself. The history of an epidemic is, 

 in the last analysis, the story of an infection with two microorganisms. 

 The epidemic ceases at the moment when all susceptible individuals 

 harbor a bacteriophage active for the causative organism of the epidemic. 

 Either the bacteriophage has acquired virulence in the body of the indi- 

 vidual who harbors it, or this individual has been "contaminated" 

 by a bacteriophage which has acquired a virulence in another individual 

 for the specific bacterium involved (d'Herelle^^^). 



