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 
