THE EVOLUTION OF INFECTIONS 41 



of sensitization throws an interesting light upon the relapses 

 so frequently noted in streptococcal infections. 



Observe what this signifies. It signifies that there is a means 

 whereby organisms that are saprophytic and non-pathogenic, 

 living upon the surface of the body, whether on the skin or 

 upon the moist mucous membranes, can become pathogenic, 

 invading and living within the tissues. What is necessary is 

 that these organisms first become habituated to growth at the 

 body temperature and then gain entrance through some solution 

 of continuity, and that, gaining entrance, they in the first place 

 induce the anaphylactic stage, so that when some week or more 

 later organisms of the same species gain entrance into the 

 supersusceptible tissue, whether through a lesion or through 

 conveyance by phagocytic leucocytes, they now are able to 

 multiply within the tissues, and multiplying acquire pathogenic 

 properties ; in other words, adapt themselves to their environ- 

 ment, gain the power of assimilating and dissociating the proteins 

 of the body, in which process they bring about the production 

 of disintegration substances, of toxic proteins or amino-acids, 

 in other words of toxines. 



But, it may be asked, if the process is so simple as this why 

 do we not observe time and again the evolution of new diseases ? 

 In answer to this question I would urge that the matter is not 

 so simple as it seems. In the first place it will be observed that 

 a particular concatenation of circumstances is requisite, namely, 

 a first entrance of bacteria into the tissues through some lesion, 

 and then, when the anaphylactic stage is at its height, a second 

 entrance of the bacteria in much larger numbers, for anaphy- 

 lactic phenomena only show themselves when the second exhibi- 

 tion of the protein (in tlnVcase of the bacteria) is larger than the 

 first, and when a definite interval of at least seven days intervenes 

 before that second exhibition. As is well shown in Pasteur's 

 method of inoculation against rabies, when a virus is inoculated at 

 successive daily intervals no anaphylactic phenomena are mani- 

 fested. And in the second place it must be remembered that 

 ordinarily the habitual growth of bacteria upon the surface of the 

 body favours the production, not of anaphylaxis, but of immunity. 

 It is by now well established that from the mucous surfaces of 

 the body bacteria are being constantly introduced into the 

 tissues, in driblets as it were, through the agency of leucocytes 



