THE BACTERIOPHAGE AND IMMUNITY 277 



resisting the bacteriophage is, by this fact, also resistant to phago- 

 cytosis. 



The bacteriophage, the direct agent of antimicrobial immunity 

 which is by its nature heterologous, at least in the sensitive 

 animal, is also indirectly an agent of organic immunity, by na- 

 ture homologous. 



The history of the disease is in effect the history of the con- 

 flict between the bacteriophage and the bacterium. We can 

 observe that the same facts hold true, but on a larger scale, 

 in the history of an epidemic. The virulent ultramicrobe, 

 which is present in the intestine of all convalescents, is dis- 

 seminated by them with their dejections. It is then capable 

 of "contaminating" susceptible neighboring individuals quite 

 regardless of whether the disease with which it is associated is 

 intestinal, septicemic, or localized in its nature. 



Observation shows that in the last analysis the history of an 

 epidemic registers the variations in the struggle between the 

 two agents, the pathogenic bacterium and the bacteriophagous 

 ultramicrobe. It is also clear that the latter is transmissible 

 from individual to individual. The immunity is contagious in 

 the same degree as is the disease itself. The beginning of an 

 epidemic is marked by the diffusion of a bacterium whose viru- 

 lence is increased progressively by passages through susceptible 

 individuals. Thus the epidemic extends. In its turn the ul- 

 tramicrobial bacteriophage increases in virulence for the patho- 

 genic bacterium, and extends equally. The epidemic ceases 

 when all susceptible individuals have been infected by the viru- 

 lent bacteriophage. 



We have seen that the bacteriophage may conserve for a long 

 time a "latent" virulence for a given bacterium. These latent 

 virulences, maintained moreover by accidental contaminations, 

 explain the difference which exists between the mode of propaga- 

 tion of sporadic diseases and of epidemic diseases. Against the 

 bacteria, agents of the first, the bacteriophage is always ready 

 to intervene, and it is only exceptionally that infection is followed 

 by disease. In the second, on the contrary, particularly since 

 the agent is most often imported, the bacteriophage does not at 

 the beginning possess a specific virulence. The epidemic extends 



