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interval may be observed and an alternation of waves extending over something
like four-thirds and two-thirds of a year commonly occurs; so that here, too, a sort
of modified annual prevalence is apt to be observed. In 1918-19 shorter intervals
still were recorded—the June-July epidemic was followed by an October-November
epidemic, and this was succeeded by a March epidemic in 1919. A very enthusiastic
advocate of the 33 weeks rhythm might perhaps claim that the October-November
prevalence was an example of the intercalation of an epidemic, halfway between
two epidemics separated by an interval not far removed from 33 weeks. The
difficulty, however, in that case presents itself that subsequent epidemics do not appear
to have followed either the June-July or March epidemics (which appeared at their
anticipated times), but, in so far as the rhythm is observed at all, follow rather at
intervals (which are, roughly speaking, multiples of 33 weeks) dating from the
time of occurrence of the intruding or "rogue" epidemic of October-November,
1918.
The clue in fact for threading a way through the maze of the varying periodicities
seems to be to recognise a normal annual interval; then alternation of intervals
of about two-thirds and four-thirds of a year, making together periods of two years;
while near the pandemic itself epidemics at even shorter intervals may ba encountered.
Study of periodogram results on the whole confirms this way of looking at the question
and thus supports the theory of three predominating factors: a climatic factor
favouring the normal annual periodicity; a varying immunisation factor which
favours shorter intervals, on either side of the mid-period between pandemic and
pandemic; and then, thirdly, a further speeding up in those years nearest to the
pandemic times; this last-named phenomenon (especially when the variation of
type which accompanies the merging of the trailer into the pandemic is borne in
mind) may be suspected to be due to a recurring biological change of state. It needs
to be realised that during all these variations, and throughout the years stretching
from pandemic to pandemic, influenza, in one or other of its phases is ever present,
perennially smouldering, "endemicbut only producing epidemics at times when
the conditions as regards immunisation, and as regards climate and season, are such,
as to permit of more or less widely spreading conflagration.
Typhoid Fever.
There were 410 cases of typhoid fever notified in London in 1924 (53 weeks),
as compared with 331 in 1923 (52 weeks). The deaths in the calendar year numbered
53 as against 42 in 1923. The increased numbers of cases and deaths were
fully accounted for by a localised outbreak in Bethnal Green in the autumn (details
of which will be found on page 35). Of the cases admitted to the Metropolitan
Asylums Board hospitals, the diagnosis of typhoid fever was not confirmed in 25.7
per cent., the corresponding percentage for 1923 being 37.9 (the latter figure iserroneously
given in last year's report as 60.9).
In 27 instances two or more than two cases were notified from one house, as
compared with 17 in 1923. Seven of the 27 instances occurred in the area (Bethnal
Green) mentioned above. The remaining 20 instances include one of a group of
four cases; nine groups of three cases, including three in which the disease was
thought to have been contracted away from London, of which one was a group of
three servants in one household ; and ten instances of two cases in a house, in two
of which the disease was probably contracted abroad, while in another instance a
third case was later reported, the patient sickening abroad. Two cases occurred
in an hotel, both being probably contracted elsewhere.
Apart from the Bethnal Green outbreak, information was received as to
probable origin of the disease in 209 instances ; in 30 it was believed to have been
contracted outside London ; in 41 fish or shell-fish were thought to have been at
fault; other foods were suspected in 15 instances; 13 nurses contracted the
disease.