MICROBIAL DISEASE AND VIRUS DISEASES 275 



which for 20 or 30 generations had shown itself to be 

 attenuated, that he accepted the idea that these varia- 

 tions depended on one single vibrio and its culture 

 medium. 



It was a great step indeed; but beyond this there was 

 nothing, and in order to see farther it was necessary 

 to consider the virus diseases. The latter presented 

 facts analogous to those of Coze and Feltz, and Davaine. 

 It was known that there w^ere benign epidemics of small- 

 pox and others that were deadly, that the severity was 

 variable in the course of the same epidemic, and generally 

 diminished as it drew to a close. It was also known from 

 the practise of smallpox inoculation, resorted to before the 

 time of Jenner, that inoculation from a benign case of 

 smallpox ordinarily produced a smallpox still more 

 benign, but this was not always true, for sometimes 

 the inoculated patient died. 



The vaccine introduced by Jenner had been a wonder- 

 ful discovery, but it had made the veil still thicker 

 behind which the virus diseases lay concealed. With 

 it variations in virulence were scarcely to be feared. 

 After being very clearly diminished in passing from 

 the cow to the man, the virulence of the vaccine was 

 maintained very constantly from arm to arm, for a 

 long series of generations. But if there was something 

 immutable in the severity of the disease or in its period 

 of evolution, there was, on the contrary, great variation 

 in the duration of the immunity which it produced. 

 So that, to sum up, the ideas which seem to us to-day 

 the most closely related, the most coherent, were at 

 that time scattered and contradictory, and no one 

 attempted to correlate them. 



It is here that Pasteur experienced the benefit of 

 his former studies and of facts which he alone knew, 

 since he had published them only in part, and in that 



