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50
Before 1905, the figures for the deaths are not strictly
accurate. Since 1905, all the outside deaths have been included
but prior to that date, a complete return of the outside deaths was
not available. Although I have not worked the figures as percentages
of the total population, they plainly show the periodicity ot
the disease. In recent years Diphtheria has made its appearand
in certain epidemic waves, the crests of which attain their maxima
at intervals of a certain number of years, although the general
tendency has been towards a diminution of the mortality. What
are the factors which influence the appearance of these waves?
I have already referred to one factor which may be operative
I have mentioned the extreme virulence of the type and it is
suggested that this is due Jp an alteration in virulence of the causal
agent. This theory would account for the outbreak by the appearance
of a very virulent type of bacillus, which has been mentioned
in a previous paragraph as the Diphtheria Bacillus Gravis. It
means that an alteration in the type of the disease has occurred in
the epidemic period. During the inter-epidemic periods the disease
is less potent and also less infectious, and it is suggested that this
increase in potency and infectivity gives rise to the epidemic.
This factor may be operative, but a more important factor
probably is the gradual accumulation of susceptible persons in the
population. This factor may act over longer or shorter periods
We have an example of the shorter period in the periodicity of
Measles, which in this country makes its appearance almost
regularly in the large towns every 2 years. The same factor
probably operates in the case of Diphtheria, but the outbreak
of a less explosive character and epidemics occur at longer intervals.
It is a matter of common knowledge that any group of
individuals contains susceptibles and insusceptibles or immunes
In the case of Diphtheria, the immunity is acquired by repeated
exposure to diphtheria infection. If a district remains comparatively
free from diphtheria for a long time, a considerable amount
of susceptible material accumulates in that community, and as
explosive outbreak sooner or later occurs. When the herd immunity
is lowered to a certain point that community is particularly
vulnerable. We do not know what rates of susceptibles cause
parasitic infections to become epidemic, and it is quite possible
that a slight raising or a slight lowering of susceptibles may make
an enormous difference in disease incidence. The measures hither,
employed against the spread of this disease which aim at the
truction of the germ—disinfection, bacteriological search
carriers, isolation of clinical cases—have proved inadequate. In
the past our mode of procedure consisted of hospital treatment of