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46. Statistics were given in the Annual Report for 1908
as to the date of calling in medical attendance in Scarlet
Fever. Some remarks were made in the same report as to
the high prevalence and comparatively low death-rates of
Scarlet Fever and Diphtheria in Woolwich.
Incubation Period. The incubation period of an infectious
disease is the period between the exposure to
infection and the commencement of symptoms. This period
for Scarlet Fever has usually been considered to be from
three to seven days, and it is often stated not to exceed
seven days. In order to trace the source of infection and
to take measures for the prevention of spread, it is very
important to know how long the incubation may last. For
this purpose I have been observing the intervals between
the occurrence of first and secondary cases in the same
house. If one child began with Scarlet Fever, say on the
first day of the month, was at once removed to hospital, and
the secondary case commenced on the seventh of the same
month, it would be justifiable to conclude that the incubation
period was six days. As a rule, however, the patient remains
at home for several days before removal to hospital,
so that it is only possible to give the period of incubation
within certain limits which may be called the maximum and
minimum periods. I have got out the minimum incubation
of 155 cases and the maximum of 296. In 76 cases (or 50
per cent.) the minimum incubation was from 1 to 7 days;
in 40 cases (or 25 per cent.) from 1 to 4 weeks; in 31
cases (or 20 per cent.) from 5 to 12 weeks; in 8 cases
(or five per cent.) it was over 12 weeks.
Out of the 315 secondary cases the maximum period of incubation
was from 1 to 7 days in 149 cases (or 48 per cent.);
from 1 to 4 weeks in 99 (or 31 per cent.); from 5 to 12