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weeks in 37 (or 11 per cent.); and over 3 months in 11
(or 3 per cent.).
Of course it is impossible to say that the secondary cases
were not exposed to other sources of infection, but it seems
extremely probable that the first case was the source of infection
in the great majority of them, and this conclusion
is borne out by the rapid fall in the number of secondary
cases occurring after the first four days' incubation, and
especially after the first two weeks. It must also be remembered
that both the primary and secondary cases may have
been due to a common source of infection, especially as
regards the cases in which the interval was under one week;
and as regards cases where the interval was longer, there
may possibly have been an intermediate case, i.e., a child
who, without having any symptoms recognised as those of
Scarlet Fever, has taken the infection from the primary case
and passed it on to the secondary case. But the occurrence of
such missed cases only strengthens the opinion that a case
of Scarlet Fever occurring in a house may be due to a
source of infection which occurred several weeks before. It
is now known that persons may carry the germs of Diphtheria
in their throat for weeks and months without
showing any symptoms of the disease, and may occasionally
after this long interval of time develop the disease. Probably
the same is the case with Scarlet Fever, and if so it is
not to be wondered at that the apparent period of incubation
is in exceptional cases as long as six months.
It would appear, therefore, from the figures given (1)
that the period of incubation varies from one day to six
months; (2) that it is barely the majority of the total cases in
which the incubation is under seven days; (3) that in a
considerable proportion of cases it is over four weeks; and