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92
The important features of these graphs are :—
(1) That the curve is essentially similar for both diseases, scarlet
fever and diphtheria.
(2) That for a number of years during the early development of
the Estate, the incidence of infection was below the average.
(3) That this period was followed by one of about three years
during which relative incidence was high.
(4) That this period was followed by a gradual return to average
incidence.
The following are possible suggestions in explanation of this :—
(1) Pure chance—The very definite line taken by the curves
and the closc resemblance of the form of the curves in both
diseases precludes this.
(2) Effect of schools.
(3) Absence of Isolation Hospital accommodation resulting in
an excessive number of secondary and return eases. As
practically all eases of diphtheria have been removed to
hospital this effect should be apparent, to any extent, only
in the case of scarlet fever.
(4) Owing to the conjunction of an exceptionally large population
at any year of age occurring with a general high epidemic
rate throughout the country.
(5) Some alteration in the herd immunity of the population.
These possibilities will now be considered in detail :—
(2) Eftect of schools—At first sight, this is a plausible hypothesis,
particularly when one realises that for the earlier years of
development—when, it is remembered, the incidence of both
diseases was low—there were no schools for the children, except for
those few who could attend the old village school. The subject
will be examined then from this aspect and from two points of
view:
(a) Whether schools in general have played any great part in
infection. If they were responsible, their effect should be
operative most in the years of highest incidence and, with
growth of school population, the incidence of infection
in the district should be higher.
(b) There should be a bias in favour of school population if
school infection caused any great proportion of the excess.