224 THE BACTERIOPHAGE 



can then acquire a virulence for the virus. If this takes place the 

 animal is thenceforth protected from the infection and becomes 

 a carrier of the virulent bacteriophage. A diseased animal prop- 

 agates his disease; an animal in a resistant condition propagates 

 his immunity. 



BUBONIC PLAGUE 



Through a lack of favorable circumstances it has not been 

 possible to follow the evolution of the intestinal bacteriophage in 

 man affected with plague. The few cases that have been ex- 

 amined have all been fatal, and at no time could the intestinal 

 bacteriophage be shown to have the least virulence for B. pestis. 

 The activity in these cases remained restricted to B. coli. How- 

 ever, the stools of two convalescent individuals have been se- 

 cured and examined. According to the physicians treating the 

 cases the material was collected on the sixth and the eleventh 

 days after the beginning of convalescence. Examination showed, 

 in the first case, a bacteriophage of average virulence (++), 

 and in the second case, one of feeble virulence (+) for B. pestis. 

 The virulence of the first of these strains has been enhanced 

 in vitro and the bacteriophage has been maintained in culture. 



An attempt was made to find a bacteriophage active against 

 this bacillus in the feces of twenty-two natives living in regions 

 free of plague, but in no case could a strain be isolated. However, 

 in view of the particular mode of infection in bubonic plague the 

 study of its propagation in man offers only a matter of secondary 

 interest, at least from the epidemiological point of view. We 

 know that an epidemic of plague in man is only consequent to 

 an epizootic among rats. That which it is interesting to study 

 is, therefore, the epizootic, the primary cause of the epidemic. 

 In order to attain a correct interpretation of results it is essential 

 to follow the natural order of things. From the point of view 

 of man the epidemic is obviously the important fact; from the 

 point of view of nature this is but a secondary incident, for if we 

 were able to suppress the epizootic the epidemic would cease 

 spontaneously. 



From what we actually know about the epidemiology of plague, 

 it results that all of the rats living in a city where there has been 



