XXI] PATHOGENIC ORGANISMS 579 



indefinite time, in some, e.g. variola, scarlet fever, and the 

 acute exanthemata in general, it seems as a rule to persist 

 for lifetime, in others, e.g.., erysipelas, diphtheria, it is of 

 a more limited duration. We said above that this naturally 

 acquired immunity is apparently not of the same character as 

 that producible by repeated toxin injections, but in reality it 

 may be the same, since also in the naturally acquired 

 immunity against a particular infectious disease by a 

 previous attack this attack is caused by the toxin elabor- 

 ated by the microbe in the infected body, so that after all 

 the difference in the two methods is merely this, that in the 

 one, the Behring's method, the toxins are prepared outside 

 the animal body in artificial cultures, while in the other, i.e. 

 the immunity acquired under natural conditions or by 

 Pasteur's method of protective inoculations, the toxins are 

 elaborated by the microbe within the infected animal 

 body. 



But is there really no difference between the immunity 

 acquired in the two methods ? We have already indicated 

 that the antitoxic power of diphtheria serum prepared after 

 Behring or Roux by toxin injections is incomparably greater 

 than the immunising, or germicidal, or antimicrobic power, 

 and it can be further shown that while an animal can by re- 

 peated injections of dead bacterial bodies be well immunised 

 and protected against an otherwise fatal dose of the same 

 bacterial bodies in a living state it is not protected against 

 an otherwise fatal dose of the specific toxin. A guinea-pig 

 is repeatedly intraperitoneally or subcutaneously injected 

 with dead cholera vibrios or dead vibrios of P'inkler, bacillus 

 coli or bacillus of typhoid, bacillus prodigiosus or proteus 

 (scraped from the slanting surface of an active Agar culture, 

 then distributed in sterile bouillon and finally thoroughly 



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