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to cases of enteric fever, separated from each other by considerable intervals, which suggest dependence
of these cases on each other, rather than on the actual cause. Again, an infected article of
food may be operative on successive occasions, the cases resulting being distributed in such a way as
to suggest infection from case to case. In these circumstances it is quite possible that an outbreak,
suggestive, so far as time and place and distribution of cases is concerned, of being in the main due to
case-to-case infection, may be actually due to infected food, infection from person to person playing
a relatively small part in the total result.
The conclusion which the investigator arrives at obviously depends in large degree upon his
views on the infectivity of enteric fever. If he regards this quality as having large potentiality he is
especially impressed by the occurrence of multiple attacks in houses which suggest case-to-case infection
from a recognised case in the house, or, in its absence, of an unrecognised case in the house, or
he may assume that the occasional visit to an infected house of a relation or a neighbour may suffice
to account for the apparent spread of the disease from one house to another. If he regards this infectivity
as having small potentiality he is especially impressed by the variability in the period of
incubation, and by the difficulty in determining the date of manifestation of the beginning of the
illness, considerations which make him think that cases with intervals between them of some
magnitude may all have been infected at the same time. Appeal is usually made to the proportion of
the persons attacked who have eaten particular articles of food, and here again the investigator
who looks upon personal infection as the dominant factor, has a tendency to be satisfied that certain
articles of food usually thought of in this connection have not been consumed by a sufficient proportion
of persons attacked to satisfy him that they have been operative, while the investigator who
attaches relatively small importance to personal infection has a disposition to direct his inquiries to
the consumption of articles of food which he thinks may be possibly operative, and to attach
importance to some article of food which has been generally consumed by those attacked.
The need for careful study of all local prevalences and for more precise knowledge of the part
which personal infection may play in the dissemination of the disease is obvious.
The following tables showing the London enteric fever case-rates and comparative case-rates at
ages in successive periods are of interest and it will be seen that the amount of decline in the "all ages"
case-rate has not been equally distributed at the various age-periods shown in the table:—