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Sweden also the less completely the ground was cleared of 
susceptible persons at any one time the sooner might the 
disease be expected to return, and after a time of compara- 
tive quiescence, so much the more virulent would it become 
on its return. 
5. It seems probable that this theory will also account for 
several pecularities in the cycles of different diseases, and 
especially for the length of the interval between their chief 
epidemics. 
Thus, cceteris paribus, the larger the range of the suscep- 
tible ages, and the longer is the epidemic period. When the 
disease has once thoroughly cleared off the susceptible 
persons at all ages, the requisite density for a great outbreak 
will be attained early or late according to the ages most 
prone to the complaint. 
I'hus whooping cough has a short cycle, and it is most 
fatal before the termination of the second year of life. It 
kills every ten years 37,000 males, and 43,000 females in 
the first two years of life, and only from 1,300 to 1,400 in 
the next two years of life. 
Measles, on the other hand, takes its chief toll at and 
under six years of age. The numbers dying in the decade 
before the fifth year of life is completed are over 40,000 of 
each sex, but the disease still kills several thousands at ages 
from 5 to 10, whilst in the next 5 years period its number 
of victims sinks from thousands to hundreds. The actual 
numbers are : — from 5 to 10 years of age, 2,900 males, 3,200 
females; from 10 to 15 years of age, 297 males, 358 females. 
It is scarcely possible to draw any conclusions respecting 
small-pox, owing to the interference of vaccination with its 
natural course, but so far as the English returns of mortality 
go, they show that the age susceptibility is very similar to 
that of measles, and we know now that its cycle is not very 
different from that of this complaint. 
Scarlet fever has the longest range of susceptible ages. 
