86 INFECTION AND IMMUNITY 



however, the power of the germs to overcome the 

 resources of nature and invade the blood or tissues 

 depends upon the toxic products developed by them, 

 they are deprived of their power to multiply in the 

 bodies of living animals when these poisonous sub- 

 stances are neutralised. Practically they become as 

 harmless as the common "saprophytic bacteria" which 

 surround us on all sides, and are swallowed in count- 

 less numbers with every glass of unsterilised water 

 we drink. 



But there is another class of substances, developed 

 during certain diseases, which exhibit specific germi- 

 cidal activity and have no antitoxic value. Such sub- 

 stances are found in the blood of animals which have 

 been made immune to the pathogenic action of the 

 cholera spirillum, the typhoid bacillus, and the bacillus 

 of hog cholera. 



The writer, some ten years ago, obtained experi- 

 mental evidence which indicates that smallpox im- 

 munity probably depends upon a substance which 

 destroys the smallpox germ, rather than upon an 

 antitoxin. 



Further details with reference to the antitoxins 

 will be found in Part Second of this volume, in which 

 questions relating to infection, disinfection, and im- 

 munity will be discussed in connection with the more 

 important infectious diseases, considered separately. 



