INTRODUCTION 171 



natural infection and we will see if the search for the bacterio- 

 phage and the determination of its properties at different stages 

 of the disease and of convalescence provides results which have 

 any relation to the pathologic condition of the patient. I have 

 selected for this investigation different infections, enteric and 

 septicemic, diseases of man and of animals, with which we will 

 show that defense by the bacteriophage is a phenomenon of a 

 general nature. Some of the diseases studied are epidemic, and 

 we will have occasion to note the effect of the bacteriophage on 

 the progress of the epidemic itself. 



If the bacteriophage is an agent of immunity, it will not appear 

 only at the exact moment when it is most needed. It should be 

 a normal inhabitant of the intestine. We will look for it, then, 

 in the healthy individual, choosing subjects throughout all animal 

 species, and this will show the generality of the presence of the 

 bacteriophage. 



Finally, we will attempt the counter-test. If, in the susceptible 

 animal the principle of antibacterial immunity resides in the 

 bacteriophage, the administration to a susceptible animal of a 

 bacteriophage active for a given bacterium ought to render the 

 organism resistant to the disease caused by this bacterium. 



Thanks to the kindness of M. Roux, Director of the Pasteur In- 

 stitute, and to M. Yersin, Director of the Pasteur Institute in 

 Indo-China, I have been able to accomplish in its entirety the 

 program which I have outlined. 



In France I have had the opportunity to study the role of the 

 bacteriophage in intestinal diseases, and during the course of a 

 year spent in the Pasteur Institute at Saigon, I have been able 

 to verify the generality of the phenomena observed, by a study 

 of a highly contagious septidemia, barbone in the buffalo, 

 and by a disease of glandular localization, plague. 



It is certain that a theory of immunity based on the bacterio- 

 phage, that is, on an autonomous organism, is so far outside of 

 all present opinion that it will stir up at first incredulity and will 

 be called a "finalistic theory," a synonym of " anti-scientific." 

 I affirm that from my point of view this theory can not be "final- 

 istic." "To be is to struggle, to live is to conquer," a very just 

 statement by Le Dantec. It is all contained in a single word 



