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26
The fact that the later horn are attacked at earlier ages
suggests, (a) the existence of a biological differentiation, or* (b)
that infection is introduced into the home by an older member of
the family suffering from discomfort or showing recognisable signs.
A general case might be stated as follows:—
Let n1 n2 represent individuals, then the whole population consists
of n1 + 2 n2 4- 3 n3 +persons. These are exposed to an extra
domestic risk. The number of families m1 m2 infected from this
source is
with 1,2,3 extra domestic cases, the intra domestic cases subsequently
arising are functions of m.
Hence the total number of cases recorded will be drawn in
greater proportions from larger families. To some extent this
theorum can be tested by comparing the distribution (a) when the
first case is notified and (b) when all cases are notified. (Notifications
during war period and notifications 1920-1921). The figures
are as follows:—