INFECTIOUS DISEASES. 625 



destroy the vitality of the pathogenic micro-organism, and also 

 by exposure to the action of certain chemical agents. 



Pasteur at once comprehended the importance of his discovery, 

 and inferred that what was true of one infectious germ disease 

 was likely to be true of others. Subsequent researches by this 

 savant and by other bacteriologists have justified this anticipa- 

 tion, and the demonstration has already been made for a consider- 

 able number of similar diseases anthrax, symptomatic anthrax, 

 rouget, etc. 



In Pasteur's inoculations against anthrax, " attenuated " cult- 

 ures are employed which contain the living pathogenic germ as 

 well as the toxic products developed during its growth. Usually 

 two inoculations are made with cultures of different degrees of 

 attenuation that is to say, with cultures in which the toxic 

 products are formed in less amount than in virus of full power. 



The most attenuated virus is first injected, and after some time 

 the second vaccine, which if injected first might have caused a 

 considerable mortality. The animal is thus protected from the 

 pathogenic action of the most virulent cultures. 



Now, it has been shown by recent experiments that a similar 

 immunity may result from the injection into a susceptible animal 

 of the toxic products contained in a virulent culture, independ- 

 ently of the living bacteria to which they owe their origin. The 

 first satisfactory experimental evidence of this important fact was 

 obtained by Salmon and Smith in 1886, who succeeded in making 

 pigeons immune from the pathogenic effects of cultures of the 

 bacillus of hog cholera by inoculating them with sterilized 

 cultures of this bacillus. In 1888 Roux reported similar results 

 obtained by injecting into susceptible animals sterilized cultures 

 of the anthrax bacillus. Behring and Kitasato have quite re- 

 cently reported their success in establishing immunity against 

 virulent cultures of the bacillus of tetanus and the diphtheria 

 bacillus by inoculating susceptible animals with filtered, germ- 

 free cultures of these pathogenic bacteria. 



In Pasteur's inoculations against hydrophobia, made subse- 

 quently to infection by the bite of a rabid animal, an attenuated 

 virus is introduced subcutaneously in considerable quantity by 

 daily injections, and immunity is established during the interval, 

 the so-called period of incubation, which usually occurs between 

 the date of infection and the development of the disease. That 

 the immunity in this case also depends upon the introduction 

 of a chemical substance present in the desiccated spinal cord of 

 rabbits which have succumbed to rabies, which is used in these 

 inoculations, is extremely probable. But, as the germ of rabies 

 has not been isolated or cultivated artificially, this has not yet 

 been demonstrated. 



