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19
to be borne in mind in drawing inferences from mere numbers.
What I mean may be best explained by an illustration or two
founded on our own local experiences within the last few years.
Thus Measles was very fatal in 1874 and 1876; the deaths were
far above the average; it was the zymotic disease that gave a
special character to those years. In 1875 and 1877, on the other
hand, the deaths from measles were below the average; but the
lower mortality, evidence of the diminished prevalence of the
disease, was the result of the excessive prevalence and fatality of
the malady in the preceding years. In saying this, I must not
be thought to ignore the fact that one epidemic of a zymotic
disease may be more severe than another; and that the fatality
of an epidemic disease may be influenced by the measures taken,
or the neglect to take any measures, to check the spread of infection.
Again, the rate of mortality in the Metropolis from
whooping cough in 1881 was the lowest on record; it is not
surprising to find, therefore, that in 1882 the disease was excessively
fatal. In our own parish the large total of 185 deaths
from this disease, in 1878, followed the minimum return of 34 in
the previous year.
Diarehœa may be cited as an illustration of quite another
kind. The mortality from this disease amongst infants was
excessive in 1878; the mortality in 1879 was much below the
average; but the diminished mortality in 1879 had no relation to
the excessive mortality in 1878. The conditions were altogether
different: the summer of 1879 was cold and wet, and, as always
happens in these circumstances, the mortality from infantile
diarrhoea was low, just as it is always high when summer is hot
and dry, as in 1878.
Again, the significance of a high rate of prevalence of enteric
fever varies widely in different circumstances. This disease may
be constantly present in a district as the result of drainage defects,
-or of a polluted water supply; whilst in another district its
introduction may be wholly accidental, as when due to casual
pollution of water, or to a specifically contaminated milk supply
introduced from without.