216 THE BACTERIOPHAGE 



A bacteriophage which has lost its virulence for the pathogenic 

 bacterium lacks the power to exercise it because of the absence 

 of this bacterium, but it possesses nevertheless, a latent viru- 

 lence. When placed again after a greater or less length of time 

 in the presence of this bacterium it regains its original virulence. 



The fact of the habitual virulence of the intestinal bacterio- 

 phage for B. gallinarum in the infected regions indicates the 

 frequency of the ingestion of these bacilli, and consequently the 

 excessive contamination of the environment by the pathogenic 

 organism. 



In contaminated regions the animal in which the intestinal 

 bacteriophage does not enjoy any activity for B. gallinarum, 

 quickly contracts the disease. It may resist and recover, but 

 this is the exception, occurring only when the intestinal bacterio- 

 phage quickly acquires a virulence for the infecting bacillus. In 

 the contrary, and usual, case the animal succumbs. 



In a chicken which recovers, the intestinal bacteriophage ac- 

 quires a considerable virulence against the pathogenic bacterium 

 and maintains this for a very long time; in fact, as long as the 

 exterior environment remains infected. This persistence of viru- 

 lence is maintained by the frequent ingestion of pathogenic or- 

 ganisms, which allow the bacteriophage to multiply at the expense 

 of the particular organism. The resistant animal disseminates 

 in its excreta the bacteriophage of enhanced virulence; the ani- 

 mals which associate with it become "contaminated" and by this 

 fact they enter the same class of resistant animals as those which 

 have recovered. Recovery of one animal in a barn-yard often 

 marks the end of an epizootic, or its arrest for a few months. 



The study of an epidemic of avian typhosis shows, in a word, 

 that the history of the contagion reflects, in the last analysis, the 

 story of the struggle between the two agents — the pathogenic 

 bacterium and the bacteriophagous ultramicrobe — and since 

 this last is transmissible from individual to individual the immunity 

 is contagious in the same sense as the disease itself. The be- 

 ginning of an epizootic is marked by a diffusion of the bacteria, 

 the end by a diffusion of a bacteriophage virulent for these bac- 

 teria. We will encounter the same facts in another disease; in 

 hemorrhagic septicemia in the buffalo. 



