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type of tlie materies morbi itself is apparently much more marked than it is in
influenza. These two peculiarities of influenza, and more especially the latter,
render study of natural immunity in this disease particularly difficult.
In attempting to formulate a provisional theory of influenza it is necessary
that the following points should be borne in mind :—
(i.) Influenza of the pandemic type stands out conspicuously from most other
epidemic diseases in respect of the great inequality and diversity of reaction manifested
in individual cases of illness ; many cases are fatal, then there are many more
of less severity, but a vast percentage of the total number are, fortunately, very
mild, or indeed so slight as to pass almost unrecognised. Again, as has again and
again been affirmed, the main stress of the disease may fall in one case upon the
cerebrospinal, in another upon the gastro.intestinal, and in a third upon the respiratory
system. This variability of type, together with the fact that the disease seems
to be specially infectious, mainly in the early hours following, or perhaps even in the
period shortly preceding, the actual development of symptoms, renders the tracing
of case to case infection difficult, and greatly complicates the task of treating by
statistical methods phenomena presented in successive epidemics.
(ii.) The trailers of influenza exhibit as a rule one main predominant type,
gastro.intestinal, cerebrospinal or pulmonary, as the case may be. While pandemic
influenza especially attacks persons of young adult or adult age, the trailers seem
by preference to affect young children or old persons who have escaped attack
during the pandemic. Correspondingly the trailers, and especially those of the
trough between two pandemic crests (i.e., those furthest removed from one or another
pandemic period), may particularly affect rural areas in civilised countries,
and elsewhere outlying thinly populated places and islands quite remote from
civilisation.
(iii.) During the pandemic phase each individual person has much larger opportunity
of infecting other persons than at other times, owing to the fact that the
population generally has as a rule outgrown any immunity it formerly possessed,
and thus nearly everyone is susceptible to attack. The average time elapsing between
contracting the disease and infecting another person is thus at a minimum, and
repeated transmission at an early period of the disease, from the infecting individual
to another individual, may be in part responsible for the change of type of the
disease—its increasing infecting power ; its acquirement of a kind of acclimatisation
to the soil in which it is growing, enabling it to flourish more exceedingly and spread
more rapidly ; and may favour also exhibition on the part of the host of tolerance
of the presence of the infective material and ability to throw off ill effects which might
at other times be expected to eventuate.
In the report on the prevalences of 1918 (Ann. Rep., 1918), the contrasts between
measles and influenza periodicities as exhibited in London were considered with
these three points in mind. It was noted that in London measles, the waves,
which are of fairly equal size, have a length from crest to crest of one and a half to
two years ; that about 180,000 children are attacked during this period, and that the
number of susceptibles in the population varies from a maximum of between
200,000 and 300,000 to a minimum of some 50,000 to 80,000. In influenza the
epidemics, instead of being of about equal magnitude as in measles, show recurring
major waves corresponding to the great pandemic prevalences, and these develop,
roughly speaking, about every 15 to 20 years ; and in addition preceding and following
each pandemic at intervals of some 1, 2 or 3 years, and showing " like ripples on
an ocean swell " there are the trailers, which present, as already noted, clinical
characteristics differing conspicuously from those observed in the pandemics.
The disparities between the simple measles curve and the complex influenza
curve may be largely accounted for apparently by the following facts :—
(a) The incubation period is roughly a fortnight in measles, and only from 1 to
3 days in influenza.