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assigned in certain cases to the incubatory period. In London, where most cases notified receive
hospital treatment (87 per cent.) in 1905, the bulk of the infection for which they are responsible must
be at the very commencement of their illness, and a large proportion of the total infection must
be at this period. It is probable, however, that a considerable proportion of infection is by
missed cases, and, as regards these, the period of greatest infectivity becomes a matter of practical
moment. Included amongst the 132 cases were eight which were missed, but afterwards established.
In each case susceptible children were exposed to infection in the house, and in six cases infection
occurred, during the first week of the infecting case in two instances, and during the second in four.
These cases seem to point to the second week as more infectious than the first, but not conclusively,
for resistance may be lowered by continued exposure. It is likely that moBt unprotected children
would succumb to sufficiently prolonged and close exposure. The families of the eight cases just
mentioned contained 20 other children under 14 unprotected by previous attack ; 11 of these, or
(55 per cent.), caught the infection, as against 19 out of 134 unprotected children in 71 families where
the first case was promptly isolated. But some children are protected by a previous undetected
attack, and some appear to be proof against any exposure.
Origin.—Most cases of which the origin can be traced are infected during the first day or
two of illness of the infecting case. A few are " return " cases. There was strong evidence for this
origin in two, and less strong in four more, of the 132 . Imperfect isolation can originate cases,
but where the great proportion are sent to hospital this must be a comparatively unimportant
factor. One such was met with. Indirect infection seemed possible in seven, two pointing
to infection by fomites after imperfect disinfection, and five to transference of infection by a child
not ill at the time. Three of these five possible " carriers " became infected later themselves, and
two are not known to have done so. These two may either have been healthy "carriers" or
very mild missed cases. There can be no sharp line of demarcation. Another possibility is infection
by a missed (or concealed) case, and the number of these is so large that they must account for a considerable
proportion of the total infection. Eight undoubted cases were met with, eleven others in
which the evidence was very strong, and six more in which the possibility was suggested. Nearly
all of these were brought to light by the onset of a second case in the family,so that the chance of
a case permanently escaping detection must be greatly increased where the family is small. The
case which remains undetected has probably not propagated the disease at home, but has every
opportunity of doing so in school, consequently the present day decrease in the size of the family
must tend to diminish home infection and increase infection in the school and street.
It may well be that the majority of those infected in the school and street are due to missed
cases. For these are very numerous, and have many of them prolonged opportunity of conveying
infection, whereas the case diagnosed at the ordinary time has little opportunity of doing so outside
its own home. Most children before scarlet fever are perfectly well on the last occasion of school
attendance. Excluding a number of doubtful cases 70 of the children dealt with were not ill in school
at all, and 21 were ill mostly at one school session only. It may be taken roughly that ten ordinary
cases make five attendances after the onset of symptoms. But ten missed cases, assuming an
average infective period of three weeks, would make whilst infectious no less than 300 attendances.
Thus the missed case has fifty or sixty times as long an opportunity of conveying infection as the
other. Many missed cases are also found in the peeling stage who have been continuously attending
school, although not in this series, where no other case can be traced to them.
The possibility of infection by a previous known case in school was established in only a
small proportion of the school cases, namely, eight out of the whole number, which followed in the
same class within a week of the last attendance of the 21 cases above referred to. Others, of course,
may probably have been infected in hall or playground. There is some evidence, however, that these
21 did cause a certain amount of infection. The day of last attendance of the 70 cases not
ill was followed within a week by a subsequent case in the same class in 16 instances against 8 in
the case of the 21 children who were ill. The proportionate number for 70 children would be 27
as against 16. The difference, and the numbers dealt with are too small to admit of any definite
conclusion. The subsequent history of a number of classes also in other schools from which cases of
vomiting were feported shows little evidence of infection from these cases. The initial vomiting
was reported from school in 47 cases, but there were only 10 cases followed in these rooms within a
week, whereas in 40 cases where there was no vomiting and the child appeared well on the occasion
of its last attendance there were 23 cases followed within a week. Eighteen cases where the child
vomited in the class room were followed by four other cases within the week.
Little evidence has been obtained that the disease is spread in school to any material extent
by cases recognised and isolated at the usual time. On the other hand, the circumstances of many
school outbreaks admit of little doubt as to the reality of a school spread from some source. In
about twenty school outbreaks about one-third showed evidence of school infection. If this is a
reality, and not due to any great extent to cases coming to light at the usual time, it might be due
largely to missed cases which are so often found on investigation, or possibly even to mere carrier
cases. Without some bacteriological means it is difficult to see how these cases are not to be missed.
There was no evidence of undue want of care on the part of teachers or parents, although one or
two parents seemed to have been lax, the atypical nature of the disease being mostly to blame in such
cases. Such atypical cases were often recognised from subsequent cases occurring in the house, or
from sequelae developing. This enquiry must be continued particularly in the direction of studying
cases only recognised in their later stages.
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