118 MICROBES AND TOXINS 



qualitative variations occur which render it necessary to take 

 into account the species of animal employed. Pasteur, after 

 exalting the virulence of the virus of rabies for the dog by 

 passage through rabbits, found that passage through monkeys 

 weakened it. The bacillus of swine-erysipelas becomes more 

 virulent (for the pig) after passage through pigeons, less virulent 

 by passage through rabbits. Passage through a foreign species 

 has been used to prepare vaccines ; thus the virus of vaccine 

 lymph undoubtedly of the same origin as that of smallpox 

 has become a "vaccine" by passage through the cow; the 

 pox of pigeons becomes a " vaccine " for pigeons by passage 

 through the fowl. In passing from fowl to fowl the spirillum 

 of Marchoux and Salimbeni gets weaker. And while it is true 

 that a feebly virulent anthrax strain can " recover " its virulence 

 by passages beginning on new-born mice, in the spirillosis 

 of fowls, on the contrary, according to Marchoux, it is precisely 

 the young of the species concerned which is the animal of 

 choice for weakening the virus and preparing an efficient 

 vaccine for the adult. 



Further, the age of the " young " individual must be 

 reckoned precisely : it is known that the new-born child is 

 less susceptible to Jennerian vaccination than the child of 

 three or four months. 



The modification of virulence may be expressed by a 

 difference in dose in relation to a " soil " agreed upon. The 

 example of Marmorek's streptococcus gives a numerical 

 measure of the increase in virulence. The spirochaete of 

 Schaudinn was inoculated from man into anthropoid apes, 

 from these into the lower monkeys, thence into the rabbit, 

 and thence into the guinea-pig ; but this has chiefly been a 

 case of progress in the technique of inoculation, for nowadays 

 it is possible to inoculate directly from man into the rabbit. 



Attenuation of Viruses. There was in Pasteur's 

 laboratory a culture of the bacterium of fowl cholera which 

 was being reinoculated daily and was of constant virulence. 

 It happened that a culture was taken for inoculation which 

 had remained untouched for several weeks in the incubator ; 



