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10
SCARLET FEVER.
Sex and Age Incidence.—Females appear to have suffered less from this disease than they
did from diphtheria. The percentage of attacks in males (to total attacks) was on an average
44 during 1914-18, 40 in 1919 and 47 in 1920. The Index Numbers (average 1914-18 basis)
were Males—1919, 68; 1920, 154. Females—1919, 82; 1920, 138.
The proportions at various ages—percentages of total cases reported—are given below.
The proportion at ages 3-15 (taken as school ages) in 1920 shows the greatest change from
76.1 per cent to 81.0 Children under three years show a considerably smaller proportion (4.0)
in 1920 as compared with 1914-18 (8.9).

A comparison of the cases reported at various ages shows a very heavy incidence at ages
13-15 and 15-25. (See below.)

Errors of Diagnosis.—In 1919 among the 261 cases notified there were 12 in which the
original diagnosis was not confirmed by the continued observation of the cases, and in 1920
among a total of 501 notified cases, 20 similar "errors." Such errors formed 4.6 per cent of the
reported cases in 1919 and 4.1 per cent in 1920, proportions notably different from those
observed during the two preceding quinquennia when the percentages were—for 1909-13, 7 4
and for 1914-18, 7T per cent.
House, Family Distribution.—Although the frequency of secondary cases in individual
households depends on factors additional to infectivity (i.e. the power of the organism of the
disease to overcome the natural resistance of the individual attacked) the variation in frequency
of such cases may be taken as a rough test of infectivity. In 1919, there were 39 secondary
cases (after correcting for "errors'') and in 1920, 71. After correcting for cases originating in
institutions, and for "errors" the secondary cases constituted 15.5 per cent of "family" cases in
1919 and 14.3 per cent. in 1920. During 1914-18 the secondary cases averaged 11.2 per cent.—
ranging from 20.2 per cent. in 1914 to 8.8 per cent. in 1917. It may be assumed therefore that
the disease had an infectivity during 1919 and 1920 between 30 and 40 per cent. above the
slandard for 1914-18. Such increased infectivity is not be confused with increased virulence,
evidence will be adduced to show that during the years under review the virulence was notably
below the average.
The recorded distribution of multiple cases in households cannot be given here as it would
occupy too much space. Below will be found the average percentage distributions for 1914-18
and 1919-20.

With the exception of houses with 4 cases each (5 only in each period) there was a
decreased proportion of each category of multiple infections during 1919-20.