THE BACTERIOPHAGE IN DISEASE 211 



These facts can be explained in only one way. A weak or 

 moderate activity of the intestinal bacteriophage for the patho- 

 genic bacterium is sufficient to render the animal resistant to 

 infection. The pathogenic bacteria which are able to penetrate 

 into the intestine are destroyed before they can multiply. But 

 it is not the same once the disease has appeared and the organism 

 is invaded. The animal recovers, and this is very rare in typhosis, 

 only because of a rapid adaptation of the bacteriophage and the 

 acquisition of a high virulence which leads to an intensive destruc- 

 tion. This bacteriophage with exalted virulence is distributed 

 broadcast with the excreta of the recovered or convalescent ani- 

 mals, and continues, indeed, during several months after recovery. 

 This bacteriophage is necessarily ingested by the other animals 

 of the barn-yard which become, in fact, "infected" by an ex- 

 tremely active bacteriophage and by this means acquire a complete 

 protection against the disease, in spite of the presence of the 

 pathogenic organism in the environment, and in spite of its fre- 

 quent ingestion, an ingestion which serves to maintain the viru- 

 lence of the bacteriophage. 



These hypotheses are not simply idle speculation, for the 

 interpretation given to these observed facts is confirmed by ex- 

 periments which provide in a controlled manner the natural 

 conditions of the epizootic. Furthermore, it will be seen that 

 the role of defense assigned to the bacteriophage is confirmed by 

 the immunization of several thousand animals by the adminis- 

 tration of cultures of an active bacteriophage. 



Before discussing these control experiments I ought to mention 

 that, thanks to the kindness of the veterinarians of different 

 regions invaded by typhosis, I have been able to procure numer- 

 ous specimens of blood and excreta taken from sick chickens, from 

 chickens which had died or from those which had recovered, de- 

 rived from eleven different foci scattered throughout all France. 

 This allows me to generalize from the facts that I have personally 

 observed. 



Control experiments 



The control experiments have been conducted in Paris, that is 

 to say, entirely outside of the epizootic area. 



