RESISTANCE TO INFECTIOUS DISEASE n 



immunity. However, the matter does not permit of such summary 

 disposal, since there appear to be other factors that enter into the 

 phenomena. The frog that does not become tetanic when inoculated 

 with tetanus bacilli or poison, develops tetanic spasms when the tem- 

 perature is raised somewhat; the hen that does not respond to an 

 anthrax inoculation develops the infection when the temperature is 

 lowered somewhat. Even for the final ingestion of bacteria by the 

 phagocytes of alien and insusceptible species the plasma principles are 

 required. 



Undoubtedly the phenomena of racial and species immunity are 

 affected by phagocytosis. But our present knowledge does not justify 

 us in disregarding other possible and contributing agencies. We are 

 still so little informed of even the grosser features of the body's metab- 

 olism that it would be premature to deny to it influence on susceptibility 

 to infection. Between the metabolism of birds and mammals there is 

 such wide disparity that an influence could easily be conceived ; but the 

 metabolic disparity is less between the herbivora and carnivora, and still 

 less between some closely related species which yet show marked differ- 

 ences in susceptibility to bacterial infection ; and as between individuals 

 of the same species it could only be the finer intramolecular variations 

 that conceivably could come into play. 



Although the properties of the defensive mechanisms of the blood 

 have not been exhausted, yet they have been defined in such detail as to 

 suffice for the moment and to permit us to turn attention, for a brief 

 space, to some of the properties of the intending invading bacteria. It 

 is matter of common experience, which each of us has suffered, that 

 the elaborate mechanisms provided for our protection from bacterial 

 infection do not always suffice, and now it becomes necessary to explain 

 why they do not. In the first place, there are very great differences 

 between the bacteria which seek to enter the body. Some species are 

 never very harmful and are readily combated, excluded or destroyed; 

 other species often possess only a moderate degree of virulence or poten- 

 tial power of doing injury and can also, as a rule, be overcome; while 

 these second species sometimes acquire such highly virulent or invasive 

 powers that the defenses prove quite inadequate to exclude or combat 

 them. During the prevalence of great bacterial epidemics it is probable 

 that this factor, virulence, plays a considerable role. Of course in 

 epidemics the bacterial causes are by the exigencies of the situation 

 more widely diffused than at other times, so that more individuals come 

 under their influence; but with even such a common bacterium as the 

 diplococcus which causes pneumonia and the bacillus which produces 

 influenza, there arise conditions in which severe and often very exten- 

 sive outbreaks, or localized epidemics, occur which are probably to be 

 attributed to an accession in virulence of these germs, although the 

 precise causes leading to the increase may not be discovered. 



Now this quality of virulence, which is often evolved so quickly 



