NINETEENTH-CENTURY MEDICINE 



"In some recent publications," said Pasteur, "I an- 

 nounced the first case of the attenuation of a virus by 

 experimental methods only. Formed of a special 

 microbe of an extreme minuteness, this virus may be 

 multiplied by artificial culture outside the animal 

 body. These cultures, left alone without any possible 

 external contamination, undergo, in the course of time, 

 modifications of their virulency to a greater or less 

 extent. The oxygen of the atmosphere is said to be 

 the chief cause of these attenuations that is, this les- 

 sening of the facilities of multiplication of the microbe ; 

 for it is evident that the difference of virulence is in 

 some way associated with differences of development in 

 the parasitic economy. 



" There is no need to insist upon the interesting char- 

 acter of these results and the deductions to be made 

 therefrom. To seek to lessen the virulence by rational 

 means would be to establish, upon an experimental 

 basis, the hope of preparing from an active virus, 

 easily cultivated either in the human or animal body, 

 a vaccine-virus of restrained development capable of 

 preventing the fatal effects of the former. Therefore, 

 we have applied all our energies to investigate the 

 possible generalizing action of atmospheric oxygen in 

 the attenuation of virus. 



"The anthrax virus, being one that has been most 

 carefully studied, seemed to be the first that should 

 attract our attention. Every time, however, we en- 

 countered a difficulty. Between the microbe of chick- 

 en cholera and the microbe of anthrax there exists an 

 essential difference which does not allow the new ex- 

 periment to be verified by the old. The microbes of 



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