Disease and Immunity 97 



resistant to the fungus which is destroying our 

 American chestnut trees. 



During the 16th, 17th and 18th centuries 

 smallpox was a terrible disease, the mortality 

 from which is said to have been "almost incred- 

 ible." With the introduction of vaccination about 

 a hundred years ago, the disease decreased both 

 in frequency and mortality. The disease is still 

 present, but as compared to the virulence of the 

 past, it is mild in form, even among the unvac- 

 cinated. The reason why a person suffers 

 only slightly from such a disease as smallpox is 

 because he has powers of resisting that disease. 

 The only way in which a person can come into 

 possession of such resistance is either by inherit- 

 ing it, or by acquiring ft through vaccination, or 

 by having the disease itself. But a person can 

 inherit such resistance only from an ancestor 

 who had it, and that ancestor could get it only 

 by acquiring it or by inheritance. 



Now it happens that this notable inherited re- 

 sistance to smallpox begins only after the prac- 

 tice of vaccination became general. That in- 

 herited resistance cannot be due to "selective 

 death-rate/* because smallpox is known to have 



