Change of Incidence of Small-Pox. 129 



To take an instance, making use of the numbers which 

 Bernoulli found to suit the case fairly in 1760, and taking 

 JIT = 24. Out of 1,000 who were living at the end of one year 

 there would be 32 subject* to small-pox at that age in the 

 one case, and about 650 similarly subject to it in the protected 

 locality, and, therefore, if small-pox attacked the latter 

 locality they might expect for a time a death rate at the 

 age of 24 about twenty times as great as the death rate at 

 a corresponding age in the locality where small-pox was 

 epidemic. In addition to this we should take into con- 

 sideration the more fatal character of the disease when 

 incident on adults, as in the latter case compared with the 

 former case, where nearly all would have suffered from it in 

 infancy. 



It must be borne in mind that no weight is laid on 

 these numbers, which afford merely an illustration founded 

 upon ratios found to be approximately suitable in 1760, but 

 the considerations involved in it may aid us in understanding 

 how the change in incidence of small-pox during the last 39 

 years shown in my tables may have originated. 



In the first place, unless other evidence of an increased 

 virulence in the nature of the disease is forthcoming, we may 

 lay that hypothesis on one side, as not being necessary to 

 explain the peculiarities shown in the tables, while an 

 increasing fatality with advancing years may be a normal 

 feature in the disease. 



Secondly, I shall take it for granted that vaccination in 

 infancy does not confer an immunity which extends beyond 

 adolescence, and consequently that beyond that age the 

 only safeguard which is general in England against epidemics 

 of small-pox is our improved sanitary condition, which is 

 again modified by the increasing density of population. 

 Judging by the general statistics of disease, we have no 

 reason to believe that our sanitary system is on the high 

 Toad to extinguish any one of our epidemic diseases. We 



