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were those of children under ten years of age. "Smallpox was in such places,"
says Dr. McVail, "a disease of childhood. . ." In London in 1884, under
general vaccination," he adds, " only 34 per cent, of the total smallpox deaths were
under 10 years," and " among the vaccinated community of London less than 9 per
cent., while among the unvaccinated community 61 per cent., were under 10
years."
In epidemic throat maladies (scarlet fever and diphtheria) the influence of natural
immunisation is very clearly apparent. Thus, in scarlet fever in London, children
under school age suffer markedly in excess in the poorest areas, and persons over
school age suffer markedly in excess in the least poor areas. In very poor boroughs
the infants of one or two years of age, being more exposed to risk of infection, suffer
more than twice as frequently from scarlet fever as those in the less poor boroughs.
Having once been attacked, they acquire immunity, and from 6 onwards to 10,
" the incidence upon children in the two groups of boroughs becomes practically
identical " ; while "after the age of ten the children in the less poor boroughs actually
suffer to a greater extent than do those in the poorer boroughs " (Annual Report for
1914, vol. III., p. 128). It was appreciated by J. Netten Radcliffe sixty years
ago (Epidem. Trans., 1864.65) that the crowded courts of the towns were the great
foci of infection, and he urged that epidemics should be controlled by improving
housing conditions, by hospital isolation, disinfection, etc. The severity, at any
rate, of the disease has, as the result of this policy, been materially lessened.
In diphtheria the position is more complex. In a paper on " Diphtheria in
older and newer Bristol" (Epidem. Trans., 1895) Dowson showed that the attack.
rate was higher in the suburbs than in the older central part of the town. This
phenomenon appeared to be mainly due, as Dowson surmised, to movement of
population from the country around into Bristol. The gradual shifting of diphtheria
prevalence over a large town like London suggests, moreover, that there is an acquirement
of immunity by one part of the town after another (see Diagram opposite p. 48,
Ann. Rep., 1918). This process has been shown to be reflected in fluctuations in the
numbers of bacillus carriers among the child population; and in New York and
elsewhere has been found to be associated with varying percentages of children
responding to the Schick test. (See p. 93.)
In phthisis the acquirement of immunity by large communities has been recently
studied by Col. Geo. F. Bushnell (" A Study in the Epidemiology of Tuberculosis,
1920"). He divides tuberculosis, as exhibited in the various countries of the world,
into two types; in the civilised community the disease is widespread but often of
chronic course, and the majority of the population become immunised ; in the more
remote parts of the world, such for example as Tropical Africa or the Islands of the
Pacific, tuberculosis is a comparatively rare disease, the morbidity rates are low, the
mortality of those that fall sick is very high. In connection with a chronic disease
like phthisis, however, it must be remembered that, as Thomas Young warned us
more than 100 years ago, we must have ever in mind the influence of migration upon
the death.rates.
The case of influenza is in certain respects comparable to that of tuberculosis—
a somewhat similar contrast between Europe and the United States, and Africa and
the Islands of the Pacific, holds good. But in influenza the manifestations of the
disease are more varied even than in tuberculosis. It is only in recent years that the
Protean character of influenza has come to be fully appreciated. Probably this is
largely accounted for by the fact that influenza is primarily a disease of the cerebrospinal
nervous system. It matters less what the precise location of a locus minoris
resistentice in the lung chances to be, so far as the clinical picture of the resulting
disease is concerned, than does the precise locus, on which the stress of the toxic
influence becomes focused, when the spinal cord or the mid.brain chances to be in
question. A millimetre's difference in the mid.brain affects the clinical picture
more than a millimetre's difference in the lung. Again in tuberculosis the stability of