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6
The fact therefore remains that there has been in recent years relative increase of incidence of
diphtheria mortality upon the ages 3—, 4— and 5— which is not explicable on the hypothesis of
transference from scarlet fever.
2. Changes due to natural behaviour of the disease.—I have already shown in my annual report
for the year 1896 that there is reason for thinking that there are seasonal variations in the age
distribution of scarlet fever and diphtheria. The following tables (VI. and VII.), showing the results
obtained from the notification statistics of the years 1892—(5 I now reproduce.

Examination of these tables, especially of the column "under 5," shows that, both in the case
of diphtheria and scarlet fever, children at the younger ages are attacked in relatively larger proportions
at the earlier and later parts of the year, and in less proportions in the summer months. There
is, indeed, in both diseases, indication of a curve of seasonal age variation. I have also shown that
the London cases of and deaths from scarlet fever and diphtheria indicate seasonal variations of fatality,
and in view of these circumstances it is impossible to affirm that the natural age incidence of diphtheria
mortality is always constant over a series of years. It is, however, obvious from the age distribution
of notified cases in August as compared with other months that there is another and important factor
concerned in determining the age incidence of diphtheria mortality.
Changes due to some social differences in the community.—This may be best considered by study
of other diseases of the same class as diphtheria. For this purpose the following tables* VIII., IX.
and X. have been prepared.
* The death rate at "all ages " is considerably influenced by variations in the age constitution of the population.