ESSAYS ON BACTERIOLOGY. 105 



such treatment affords. Thus Pasteur's first vaccine 

 aflfords but feeble protection, the second vaccine in- 

 creases the security; and so advancing, step by step, 

 from weaker to stronger virus, practically absolute 

 protection against rabies is secured to animals. 



Fourth. The protection afforded by one attack of 

 a disease is a positive, an active thing ; it is an endow- 

 ment with a new power. 



He who has not had measles, scarlatina nor variola 

 is susceptible to those diseases. In no other known 

 way can he obtain an immunity. If this statement 

 seems trite and unworthy of the prominence here 

 given it, the justification will be found in its impor- 

 tant bearing upon the practical application of vaccina- 

 tion. 



If now these principles are correct, they ought to be 

 applicable to vaccination. 



First, we should not expect the protection thus af- 

 forded to be absolute. Experience alone could decide 

 the matter. And experience leaves us with a trifle of 

 uncertainty, because the immunity approaches so near 

 to completeness that we can but suspect that the few 

 exceptions are only apparent and due to some defect 

 in the vaccination. Those few persons who have had 

 variola 'within a few years of vaccination may and 

 probably have not had a genuine inoculation with 

 cowpox. But, it need hardly be said, the departure 

 from complete protection is so slight as to be of small 

 practical moment. 



