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99
The graph shows that the majority of cases of infection appear
after a period of residence on the Estate of some six months to two
years. This is particularly marked in the case of scarlet fever, but
appear similarly in the case of diphtheria. After a period of two
years, the cases of infection in relation to the period of residence,
appear roughly in proportion to the susceptible population.
This grouping suggests that there is some cause which operates
after a period of residence here which renders the person more
susceptible ; that is, that after a period of residence in this district,
the population loses its existing immunity.
This suggestion is supported by a consideration of the incidence
of infection on the various sections of the Estate in relation to
their period of development. Table X shows for each section the
time occupied in the erection of the section and the number of cases
occurring in succeeding years. Table XI shows the incidence by
quarters of the year in each section. It will be noticed that there
is a preliminary period of lag of some months in most cases before
any infection appears. After this, in the more slowly built sections,
there is a slight incidence for a number of years, replaced later by a
heavier incidence. In the sections more rapidly erected, however,
although there is a period of lag demonstrated, a heavy infection
makes its appearance much earlier on, in some cases appearing in
the second year of development.
That there is a relatively increased susceptibility in this
population is demonstrated by the, admittedly few, Schick Tests
carried out. Of children of seven years of age and under, two
were negative out of 20 ; of those of ages eight and nine there were
six immune out of 17 and 10 out of 26 of those over 10 yearsof age.