106 ESSAYS ON BACTEEIOLOGy. 



Second, we should not expect the protection of vac- 

 cination to be permanent; certainly not in view of the 

 fact that -unmodified smallpox itself does not always 

 afford permanent safety. Jenner himself erred at 

 'this point, believing that cowpoxing protected for life, 

 and we can easily understand how he made this mis- 

 take. With the light of modem knowledge, still 

 more with the light of a century's experience before 

 us, we can easily pardon an error into which he fell 

 simply from lack of experience which could only come 

 with the lapse of years. He did not live long enough 

 to get a final view of the matter. When, with pass- 

 ing years, some of those who had been vaccinated be- 

 gan to show a susceptibility to smallpox, and the truth 

 which is so clear to us began to force itself upon the 

 world, it came with such a shock as almost to threaten, 

 as I have said, the ruin of vaccination. The suspicion 

 arose and was industriously fostered that the whole 

 thing was a gigantic and astounding delusion. For- 

 tunately, calmer minds saw the true state of the mat- 

 ter, with the result that the necessity for and the prac- 

 tice of re-vaccination became established. 



While it is evidently impossible to fix upon a time 

 at which the eificacy of vaccination has declined to the 

 danger point, there is a substantial agreement among 

 all authorities which, being acted upon, leads to al- 

 most assured safety. The German statistics thor- 

 oughly and accurately collected seem to show that no 

 case of smallpox has occurred in that country within 



