EIGHTH PART 

 The Study of Viruses and Vaccines 



]\nCROBIAL DISEASES AND VIRUS DISEASES 



What idea could one have, about 1880, of virulence 

 as we understand it to-day? The answer is easy if one 

 is willing to regard it from the point of view of our 

 actual knowledge. Considerable variation in virulence 

 had been determined in various microbial diseases, 

 but it was not known whether they had the character 

 of the virus diseases, that is, would not recur in the same 

 individual. For the true virus diseases, such as small- 

 pox and cowpox, the established variations in virulence 

 were feeble, and supposed to be dependent, as we have 

 seen, on external conditions, which amounts to saying 

 that they were unexplained. 



This point of view shows us clearly the difficulties 

 of the question, but it is not from this standpoint that 

 out observations should be made. What could one 

 think at that time concerning the relation between 

 microbial diseases and the virus diseases, is what 

 we must ask ourselves. Strange to say, one thought 

 nothing! These were two territories separated by an 

 arm of the sea over which there was no bridge. From 

 one of the continents a person might indeed from time 

 to time glimpse the other and observe its outlines, 

 but both seemed isolated and equally impenetrable. 



For Pasteur alone, the man of the large horizons, 

 they were in some places in contact. The careful 



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