244 LOUIS PASTEUR. 



immunity generally lasted longer than a year. It is, 

 however, prudent to vaccinate every year, and to 

 select for performing the operation a period when 

 splenic fever has not yet become developed in March 

 and April. If the vaccinating is postponed until the 

 fever is in the sheepfolds, there is the risk of attri- 

 buting to vaccination the losses which in reality 

 belong to the natural disease. Just as human vacci- 

 nation cannot preserve from small-pox a patient who 

 is already under the influence of small-pox, so the 

 splenic vaccinations are powerless against a fever 

 already in process of incubation. 



It must not be assumed that the duration of im- 

 munity to animals after splenic vaccination cannot be 

 compared with the duration of immunity from small- 

 pox after Jennerian vaccination. Jenner and his con- 

 temporaries believed that vaccination was able to 

 preserve from small-pox during the whole life. That 

 illusion disappeared long ago, and now ten years has 

 been fixed as the average duration of that immunity 

 and of the interval which ought to separate successive 

 vaccinations. This interval, moreover, is too long for 

 a certain number of individuals. Besides, in order to 

 judge of the immunity of antisplenic vaccination, we 

 must not lose sight of the formidable trial which 

 vaccinated animals have to undergo when inoculated 

 with the most virulent virus. What doctor would 

 dare to subject a vaccinated child to inoculation from 



