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105
would render them in a position to succumb to infection at some
period after their original date of taking up residence here.
The absence of heavy infection in the early years would be
attributed to relative lack of facilities for conveyance of infection
to the population. With the extended development and greater
congestion, with more communication of residents amongst
themselves, there would be increased exposure of the susceptible to
infection. Those sections erected at the more rapid rates would,
no doubt, suffer more, as larger volumes of population would be
rendered susceptible at the one time. Also the greater crowding
of the section in itself, and its proximity to other sections, would
permit of greater possibilities of transmission of infection.
The relative excess of infection occurring over a period of time
will result in an alteration in the herd of immunity. Not only will
the susceptibles be reduced by a number equal practically to the
number who succumbed to infection, but there will be the effect
of each patient contributing, to a greater or less extent, his quota
of infectious material to others, resulting in some cases in another
infection, but in others in some degree of immunity.
By these means, the herd immunity index will gradually be
restored to normal, with resulting average incidcncc of infection.
It appears that the cxcess infection which has occurred in some
years, is determined by a number of factors, some at least of which
arc under control. The prevailing type of scarlet fever is mild,
and little harm appears to have resulted from the infection of these
children. Should the type of scarlet fever illness revert, as it
probably will in a number of years, to that which prevailed in this
country over a generation ago, the acquisition of some degree of
immunity as result of this mild infection, might even result in
benefit, by saving the individual from a serious illness in the future.
Diphtheria, however, even to-day, is a serious disease and the
average fatality rate occurring over the greater number of persons
resulted in additional deaths. Ilad the prevailing type of scarlet
fever been that occurring at present in South-East Europe, the
occurrence of such a large number of cases would have been a
matter of grave concern.
Since the increased susceptibility appears to be the result of
the loss of immunity occurring through the deficiency of healthy
carriers, appearing most markedly in districts in which the entire
population is in the same state of susceptibility, some preventive
measures are possible.