1905-6.] Studies in Immunity : Theory of an Epidemic. 489 
Between the conditions which existed in earlier and later times 
there is some difference. Then smallpox might be entirely absent 
from a district for long periods of time, or might recur in epidemics 
every three or four years. These two conditions present differences. 
Diagram III. 
1 
Lfinn 
1 1 
Influenza Deaths. 
ruuu 
Lyi nn 
/ / 
/ / 
/ 
jl 
v 
\ 
\ 
London, 
1891. 
- onn- 
\ 
4UU 
j 
A p.13 Ma‘ 
y 2 May 16 Ma 
i 
y 30 Jun 13 Ju 
n. 29 Jul. 11 Jul' 
! J 
!, 25 Aug. 8 
In the former, with half the city open to infection, as in Boston in 
1721,* a bad epidemic might burn itself out partly from absence of 
material. In the latter, those susceptible would he more or less 
thoroughly mixed with the unsusceptible, and the chance of infec- 
tion reduced so that the organism would have a better opportunity 
of producing a typical epidemic. The latter approximates more 
closely to the condition seen at present, where vaccination provides 
a large insusceptible population. Of the former, the epidemic in 
Boston, U.S.A., in 1721 (diagram VI., Table A, No. 9), may be 
* Creighton’s History of Epidemics, vol. ii. p. 485. 
