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102
this out as an explanation. The large number of children, as such,
will not result in any increase, any more than should the same
number of children in another district. This area in 1927 had an
undev-15 population of 18,750 out of the total population of 48,850.
A population with normal age distribution would have that number
of susceptibles in a total population of 75,000. The 18,000
susccptibles should contain no more cases in the one than in the
other district, if the sole factor in dissemination were the actual
susceptible population. Considering the balance of the population,
however, it is seen that in this area, the number over 15, that is the
presumably immune, are only 1times the number of susceptibles;
whereas in a population of ordinary age distribution, the immunised
are roughly three times as common as the susceptibles.
This low immunity index of the population, by which is taken
to mean the proportion of immunes to susceptibles, is probably the
determining factor in introducing this increased rate of infection.
The question is, in what manner does it produce its effect ?
A suggested explanation is the increased contagiousness of the
organism as a result of its rapid passage through a susceptible
population. There is no evidence to support this.
Another suggestion is that there is a general increase in the
susceptibility of the population, or to put it the other way round, a
loss of what immunity had previously been acquired.
It is acceptcd, in general, that lack of susceptibility to infectious
diseases shown at increasing age, is not due to the effect of age
per se, but that it is the result of tissue reaction to repeated
subliminal doses of infection.
The result of Schick tests show that, in diphtheria, there is a
period of a few months during which most children arc immune,
rom six months, at which practically all are susceptible, to the
age of 10 or 12, there is a steady fall in the percentage of
susceptibles. In congested districts the figure at this age may
reach 50%, falling to about 30% for adults. In more rural
communities the decline is present but does not reach so low a
level. There is, however, no marked deviation in the rate of
percentage diminution of susceptibles at or about the age of
admission to school.
The question is, where does the child receive the subliminal
doses that result in the protection. Schools are blamed to a
greater or lesser extent for the production or spread of cases of
infection action. In the case of the modern elementary school, it not run
under conditions of overcrowding, there seems little justification