PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 227 



bactericidal forces of the body, bears the specific pathogenic char- 

 acter of the microbe. It is the substance which, according to the 

 nomenclature of Ehrlich, calls forth the amboceptor from the re- 

 sources of the host to combine with it, and thus open the way for 

 the usual bactericidal forces or complements according to Ehrlich. 

 The existence of this specific protective body will account for the 

 varied resistance of animals to the same micro-organism and the 

 relative difficulty to induce immunity. The more difficulty the 

 body has in producing the amboceptor, the greater the difficulty 

 in acquiring immunit}\ 



In the departments of preventive arid therapeutic medicine, the 

 isolation of this protective substance apart from the body toxins 

 would be of prime importance in combating disease by inducing 

 individual resistance. In fact, the theory that the so-called immun- 

 izing and disease-producing substances are separate is not new, 

 but has been presented under various forms. The tendency to 

 give up the toxic extracts of bacteria, and use the latter in their 

 entirety in immunization, pays tribute to these unknown bodies. 

 The most prominent example of this change was the abandonment 

 by Koch of the old tuberculin, a boiled extract, and the utilization 

 of the entire tubercle bacilli, ground and uninjured by heat, in the 

 production of immunity in tuberculosis. 



The foregoing hypothesis, that the tendency of microbes in per- 

 fecting the parasitic habit, is to act solely on the defensive, is to 

 a certain degree supported by a phenomenon of considerable bio- 

 logic importance, which I wish to discuss very briefly. 



If we examine the statistics of the various infectious diseases we 

 are struck with the relatively low mortality of most of them. There 

 are few highly fatal plagues now known. To be sure, the mortality 

 of many infectious diseases is regarded as formidable by sanita- 

 rians, but if we disengage ourselves from the humane view for the 

 moment, and take the biologic standpoint, we will agree that the 

 relatively high mortality of 25 per cent to 50 per cent indicates a 

 very decided preponderance of the resisting powers of the human 

 race. The odds are always against the invading microbe. This state 

 of affairs appears for the moment to contradict the results of ex- 

 perimental bacteriology, which teach us that the virulence of mi- 

 crobes may be more or less rapidly raised by repeated passages 

 through susceptible animals, or even through those which possess 

 considerable resistance. The accustoming of bacteria to antiseptics, 

 bactericidal and agglutinative serums, has already been mentioned. 

 With this capacity for adapting themselves to the defensive mech- 

 anisms of the host, why should not the infectious diseases become 

 more, rather than less, virulent? What is it that keeps their viru- 

 lence on a low level? This problem has occupied my attention for 



