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3. The child population resident in congested areas in
London is in a partially immunised condition the extent
of immunity increasing with advancing age. On coming
to reside here and being deprived of further sub-lethal
doses of infection, their immunity is lost. After this
they are in a position to succumb to infection.
4. In the early years of the development of the Estate, the
rate of growth was, compared with later years, slow
Further, building was started at different points Separated
from each other. In these circumstanees there would
not be any one large section of population becoming
susceptible at once. As, after a period of months, a
certain proportion lost their immunity, they would be
merged in with the new-comers who at that time would
still be relatively immune. More recent development
however has taken place at a greater rate and in sections
contiguous to already developed areas. The circumstances
would now be altered in that larger sections of
the population would become susceptible at the same
time, without the balancing effect of an imported population.
Such sections will, after a period during which
loss of immunity occurs show a relative excessive incidence
of infection.
5. The introduction of infection might well be through the
medium of Visitors to London. The.iC persons become
recognised or unrecognised cases or temporary or chronic
carriers and bring infection to the susceptible population
who then succumb.
In support of the various items of this hypothesis:—
1. Whilst it is admitted that active acquired immunity
developed as a result of exposure to sub-lethal doses
infection, it is unknown what this source of infection is
Active cases of illness, when recognised, arc segregate
so that although posibly active in the producation of
immunity amongst immediate contacts, just as they are
liable to cause infection amongst these persons they
cannot have much effect on the general population
That the. proportion of immunes rises in a semi-isolated
community after the occurrence of cases does not of
necessity require that the active cases of disease should
have been instrumental in causing this immunity
cases might have given rise to temporary healthy carries
who thereupon pass on immunising doses to " other
Chronic carriers, particularly nasal, are a source of
infection to a wide circle. They can, presumably,be