BACTERIA IN INFECTIOUS DISEASES. 241 



death by the mechanical effects which result from 

 its presence in such vast numbers, i. e., by blocking 

 up the capillaries and the formation of emboli ? 



There can be little doubt that, in these acute 

 infectious diseases, the parasite injures its host in 

 all three of the ways indicated, and that a fatal 

 result is to be ascribed to the three causes men- 

 tioned conjointly. 



A most difficult and important question in con- 

 nection with these diseases is that which relates to 

 the rationale of the immunity produced by protec- 

 tive inoculations practised by one of the methods 

 described in PART FOURTH of the present volume. 

 In these protective vaccinations, the virus used is 

 either greatly diluted or is modified as regards the 

 reproductive activity of the parasite by exposure 

 to oxygen, by heat, or by certain chemical re- 

 agents. A susceptible animal, when inoculated 

 with virus " attenuated " by one of these methods, 

 does not succumb to the attacks of the parasite, 

 but, after experiencing a mild form of the disease, 

 recovers, and is subsequently protected from the 

 effects of full doses of unmodified virus. 



This recovery after inoculation with attenuated 

 virus is more easy to understand than is the subse- 

 quent protection. There is evidently some pro- 

 vision of nature by which invading organisms may 

 be disposed of when they do not multiply too 

 quickly, but which fails when they have very 

 great reproductive activity, and when the con- 

 ditions within the living animal are extremely 



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