Pasteur's Treatment 467 



Inasmuch as the toxins of diphtheria and tetanus circulate 

 in the blood, and not infrequently saturate the nervous 

 systems of affected animals, it might be inferred that the 

 material thus producing the rabies is a toxin. This is 

 readily disproved, however, not only by the fact that a 

 toxin would weaken instead of strengthen by transfer 

 from animal to animal, not being vital, but also by the 

 fact that when such an emulsion of the nervous system 

 of an affected animal is filtered through porcelain, its 

 virulence is entirely lost. This seems to prove that the 

 disease depends upon a living, active body a parasite, and 

 in all probability a bacterium. However, all endeavors to 

 discover, isolate, or cultivate this organism have failed. 



Pasteur observed that the virulence of the poison was less 

 in animals that had been dead for some time than in those 

 just killed, and by experiment found that when the ner- 

 vous system of an infected rabbit was dried in a sterile at- 

 mosphere its virulence attenuated in proportion to the 

 length of time it was kept. A method of attenuating the 

 virulence was thus suggested to Pasteur, and the idea of 

 using it as a protective vaccination soon followed. After 

 careful experimentation he found that by inoculating a dog 

 with much attenuated, then with less attenuated, then with 

 moderately strong, and finally with a strong, virus, it 

 developed an immunity that enabled it to resist infection 

 with an amount of virulent material that would certainly 

 kill an unprotected dog. 



It is remarkable that this theory, based upon limited accu- 

 rate biologic knowledge, and upon experience with very few 

 bacteria, should find absolute confirmation as our knowledge 

 of immunity, toxins, and antitoxins progresses. What Pas- 

 teur did to produce immunity against rabies is what we now 

 do in producing the anti-serums i. e., gradually accustom 

 the animal to the poison until its body-cells are able to 

 neutralize or resist it. As in the case of rabies the specific 

 poison cannot be cultivated outside of the body, because the 

 bacilli, micrococci, or whatever they may be, had not yet been 

 discovered, Pasteur introduced the unknown poison-pro- 

 ducers, attenuated by drying, and capable of generating only 

 a little poison, accustomed the animal first to them and then 

 to stronger and stronger ones until immunity was estab- 

 lished. It was upon this same principle that Behring sub- 

 sequently began his work upon diphtheria immunization. 



