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Of these 181 all but three were removed to hospital, these
three remaining at home at the parents' request in favourable
home circumstances.

30
ceptible host might acquire greater pathogenicity or virulence
than if passed in stages through an immune intermediate host.
As compared with more stable districts, this one with its
rapid growth is possibly influenced in another manner. Each
week sees the addition to the population of some hundreds of
inhabitants, these additions on the whole being, because of their
age distribution, more susceptible to infectious disease than a
normal population. This fact becomes important in view of the
conclusions rcached by Topley as a result of his work on the
spread of disease amongst mice.
"... continuous immigration of susceptibles appears to be
the most important single factor in maintaining the level of
epidemic prevalence of an infective disease," and the suggestion
that " it would appear to be an unfortunate occurrence for individuals
forming part of an infected herd to be subjected to
immigration from without that herd, of other healthy individuals
of their own species."
Diphtheria.
181 cases per 50,000 is at the rate of 3.62 per thousand living,
the corresponding figure for England and Wales for 1927 being
1.33, i.e., Diphtheria is apparently 2.7 times as prevalent in
this area as in the country as a whole. The incidence rate for
the country per thousand children under 15 is 5.32. For this
district, assuming that there are 18,750 children under 15, the
corresponding figure is 9.1, i.e., when allowance is made for
the increased proportion of susceptibles in the population and
comparison is made, not per thousand population, but per
thousand susceptible population, Diphtheria is seen to be 1.7
times as prevalent as in the country as a whole.
In the earlier months of the year there were but few cases
of Diphtheria in the district. From April 1st to June 4th there
were only 14.