SMALLPOX AND COWPOX. 649 



redness and swelling at the point of inoculation, with 

 some general disturbance. After the passage through 

 several animals an affection exactly similar to cowpox 

 occurs. The successful inoculation of the first series 

 of cattle from smallpox is a matter of great difficulty, 

 but so many experimenters have asserted that they 

 have produced lesions similar to cowpox from small- 

 pox that there seems no possibility of doubt that it 

 has been done. In the laboratory we have failed in 

 several attempts. 



Experiments have demonstrated that children vacci- 

 nated with cowpox vaccine are not susceptible to inocu- 

 lation with smallpox lymph, and also that those who 

 have passed through smallpox cannot be inoculated 

 successfully with cowpox vaccine. The mutual immu- 

 nity conferred by inoculation with either, the similar 

 appearance of the bodies in the cells about the vesicles 

 of both, and the statements from reliable sources that 

 smallpox virus has produced in cattle a disease indis- 

 tinguishable from cowpox, leaves hardly any doubt that 

 the two are due to the same micro-organism, which has 

 become modified by transmission through cattle. Why 

 such passage should produce a permanent change in the 

 virulence of the organism is undoubtedly a difficult 

 matter to explain, but we must remember that we 

 know practically nothing about the life-processes of 

 this form of micro-organisms, and changes once pro- 

 duced in them may tend to become fixed. 



The Duration of the Immunity Conferred by Vaccina- 

 tion. The immunity caused by successful vaccination 

 is not permanent, and varies in its duration in different 

 individuals. Although it may give some protection 

 from smallpox for ten or fifteen years, it is not well 



