56 INFECTION AND RESISTANCE 



gross physiological differences such as body temperature, the factors 

 determining species resistance are largely a mystery, and in the 

 matter of racial variations, of course, we have no instances in which 

 such very obvious physiological factors play a part. In attempting 

 to find causes for differences of resistance or susceptibility in gen- 

 eral, the nature of the problem makes it necessary for us to examine 

 it from a number of different points of view. A micro-organism 

 may be infectious for a given species of animal more than for 

 another, because of special adaptation to the conditions, nutritive 

 and otherwise, encountered in the tissues of these animals. Such 

 adaptation is illustrated in the experience of Pasteur with "rouget" 

 and with rabies, where passage through one variety of animal en- 

 hanced the virulence for this species but reduced it for others; and 

 the same thing is easily demonstrated in the laboratory with so many 

 bacteria that it may be accepted as a principle underlying enhance- 

 ments of virulence in general. This adaptation implies that, to a 

 certain extent, the part played by the animal body in determining 

 its own susceptibility is passive. Gonococcus, for instance, infec- 

 tious for man only, requires human protein for growth, at least in 

 its first generations outside the body. Its ability to cause disease 

 in man may be largely dependent upon its cultural need of human 

 protein. The resistance of other animals to this disease, then, is, in 

 part, due to their failure to supply proper nutriment. This, as Kolle 

 points out, is analogous to Atrepsie, a term used by Ehrlich, in 

 speaking of the insusceptibility of one species to cancerous growths- 

 originating in another. 



Again, "adaptation" on the part of the bacteria may imply, not 

 only an increased ability to meet altered cultural conditions, but an 

 actual acquisition of greater offensive or invasive powers with which 

 to meet the particular defences opposed to it by the given animal. 

 Thus the increased virulence of typhoid bacilli after cultivation in 

 immune sera would point toward an increased ability to survive 

 under the adverse conditions encountered in the animal body. An 

 organism may possibly acquire particular infectiousness for one 

 species to the exclusion of others, by a succession of spontaneous 

 inoculations comparable to the experimental passage of the micro- 

 organism through animals of the same species. This is especially 

 probable in diseases such as gonorrhea, syphilis, and some others 

 where infection is usually direct from one person to another. And 

 it is these diseases particularly in which infectiousness is rather 

 strictly limited to the human species. 



Regarding the matter purely from the point of view of the ani- 

 mal body and the factors which determine its powers to ward off a 

 given infection, we may justly assume that natural resistance may 

 be largely a matter of inheritance. Whether this is to be interpreted 

 as purely an instance of survival of the fittest or whether immunity 



