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21
preventible character; it being customary, moreover, to regard the
absence or the prevalence of certain of them as a test of the
sanitary condition of a district. But, without underrating the
importance of this test, it must be said that there are limitations
to its applicability 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 an 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; or 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.
Diarrhœa 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 happened in 1878.
Again, the significance of a high rate of prevalence of enteric
fever varies widely in different circumstances. This disease may