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fatality 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 severity
of an epidemic may be influenced by the measures taken, or
the neglect to take measures, to check the spread of infection.
Again, the number of deaths from Whooping-cough in 1889
was the lowest on record; it is not surprising to find, therefore,
that in the immediately preceding and succeeding years
this disease was fatal above the average : the large lotal of
185 deaths in 1878, moreover, followed the then minimum
return of 34 in the previous year.
Diarrhcea may be cited as an illustration of quite
another kind. The mortality from this disease, amongst
infants, was excessive in 1887; the mortality in 1888 was
much below the average; but the diminished mortality in
1888 had no relation to the excessive mortality in 1887. The
conditions were altogether different: the summer of 1888
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 the summer is hot and dry, as in
1887.
Again, the significance of a high rate of prevalence of
Enteric Fever varies widely in different circumstances. This
disease may be constantly present in one district, as a result
of drainage defects or of a polluted water supply; whilst in
another district its introduction may be wholly accidental, as
when it is due to casual pollution of water, or to a specifically
contaminated milk supply introduced from without.
These and like circumstances must be kept in view if we
would draw sound conclusions from a high or a low rate of
prevalence of zymotic diseases, particularly in relation to the
sanitary condition of a district.