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It is to be regretted that for the purposes under consideration cases of measles and whoopingcough
are not notified in London, and hence the seasonal age distribution of cases and fatality of
these diseases cannot be shown in the same manner as in scarlet-fever and diphtheria. It is,
however, possible to ascertain whether there is evidence of seasonal variation in the age distribution
of the deaths from these diseases, and to compare any such variation found with that of
scarlet-fever and diphtheria. Owing to the comparatively few deaths from scarlet-fever and
diphtheria of children under one year of age, and to the fact that nearly all the deaths from
measles and whooping-cough occur among children under five years of age, it is necessary in this
connexion to consider the proportions of deaths under five years of age from the first two of these
diseases, and under one year of age from the last two. The following tables, showing the monthly
age distribution of deaths from scarlet-fever, diphtheria, measles, and whooping-cough in London
in the period 1889-98, have been prepared for this purpose, and it will be seen that while in
scarlet-fever and diphtheria deaths among children aged 0—5 attain a maximum proportion in
April, after which this proportion decreases during subsequent summer months (with the usual
interruption in August), in measles and whooping-cough among children aged 0—1 the position is
practically reversed. The age distribution of the deaths is no doubt dependent in the main on the
age distribution of the cases, and examination of the curves of age distribution of cases and deaths
of scarlet-fever and diphtheria lead to this conclusion. It may be anticipated, therefore, that
notification of cases of measles and whooping-cough in London would show seasonal variation in
the age distribution of the cases, but that the curves, at ages 0—1, would be entirely different
from the curves at ages 0—5 in the case of scarlet-fever and diphtheria.
It is interesting to observe that the months when the cases and deaths at the younger ages
form the largest proportion of total cases and deaths are not the months of greatest mortality
at all ages from these diseases. In other words, seasonal curves, based upon the proportion which
cases and deaths at the younger ages constitute of the total cases and deaths are different
from seasonal curves based upon total cases and deaths. There appears, therefore, to be some
condition, possibly quality of disease, which is not constant throughout the year, and which in
certain months gives the disease a greater power of diffusion, enabling it to attack, in larger proportion
than in other parts of the year, persons of older age.