THE BACTEEIOPHAGE AND IMMUNITY 275 



the pathogenic bacterium grows and disease results. If the en- 

 vironmental conditions remain unfavorable and inhibit the 

 action of the bacteriophage, the bacterium develops freely and 

 the invaded individual succumbs quickly, or the conflict may 

 become established, with the virulence of the bacteriophage 

 gradually increasing by selection and the resistance of the bac- 

 terium likewise increasing as the result of a similar selective 

 process. The condition of the individual in which this con- 

 flict is taking place faithfully reflects the changes in the 

 struggle. Convalescence is established only at the moment 

 when the virulence of the bacteriophage effectively dominates 

 the resistance of the bacterium. If the opposite results, if the 

 bacterium acquires a refractory state, no further barrier is opposed 

 to the invasion of the individual and death follows. 



In a word, recovery is always a result of the exaltation of the 

 virulence of the bacteriophage, an increase sufficient to permit 

 it to parasitize and destroy the pathogenic bacteria implanted 

 in the body. Death takes place, either as a result of inertia in 

 the bacteriophage, or because of the acquisition of a refractory 

 state by the bacteria, conditions which, in either case, allow the 

 latter to develop without hindrance. 



There is, however, a third possibility. One may isolate from 

 the intestinal contents of " bacillus carriers," typhoid or dysen- 

 tery, and indeed constantly, a resistant pathogenic bacterium and 

 a bacteriophage virulent for this bacterium. There has been 

 a commensality (as is the case with the normal saprophytes of 

 the intestine), a mixed culture. Usually this equilibrium is 

 quickly broken in favor of the bacteriophage and the carrier state 

 ends. But in individuals who become carriers the conflict car- 

 ried on in the intestine between the bacteriophage and the bac- 

 terium is sufficiently long to permit the development of an organic 

 immunity. The bacteria act on the organism only by means 

 of their toxins. A bacterium whose toxin does not exercise 

 any action on the cells of an animal is as inoffensive for it as a 

 bacterium naturally atoxic. From the time when an organic 

 immunity is acquired by the carrier the pathogenic bacterium 

 becomes for him a saprophyte. 



One can comprehend, on the other hand, the danger of con- 

 tamination from carriers, who, although they distribute a viru- 



