228 THE BACTERIOPHAGE 



CONCLUSIONS 



We may limit ourselves for the moment to the following points: 



Whatever may be the disease considered, the picture remains 

 the same; when a pathogenic bacterium is introduced into an 

 organism, one of two situations develops: 



First. The intestinal bacteriophage shows an activity for 

 this bacterium, the latter is destroyed before it can develop, and 

 disease does not appear. 



Second. The intestinal bacteriophage is inactive, the bac- 

 terium develops, and disease results. 



In the course of the disease one of two things may happen: 



First, the bacteriophage in contact with the pathogenic bac- 

 terium may acquire a virulence, or 



Second. The bacterium may acquire a virulence, in other 

 words, may become resistant to the action of the bacteriophage. 



The vicissitudes in the struggle between these two factors are 

 reflected in the condition of the infected individual. Convales- 

 cence begins at the moment when the virulence of the bacterio- 

 phage is sufficient to give it, definitely, the upper hand. The 

 disease has a fatal outcome if the bacteriophage is inactive as a 

 result of unfavorable conditions, or if the bacterium is able to 

 acquire a refractory state. This last situation appears to be 

 very infrequent, at least, in the diseases studied. 



In epidemics we find a large scale reproduction in a community 

 of individuals of the struggle which takes place in a single indi- 

 vidual between the ultramicrobe and the bacterium. 



The bacteriophagous ultramicrobe is transmissible from one 

 individual to another just as is the bacterium itself. The his- 

 tory of an epidemic is, in the last analysis, the story of an infec- 

 tion 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. 



