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pulmonary complications among old persons and children, and the nervous complications in children,
will render the case mortality much higher. The first "trailer" will not spread so quickly as does
the pandemic prevalence, partly because its predecessor skimmed off the cream, as it were, of the
susceptible material (i.e., of those who mix with the world and are thus specially likely to be attacked)
and partly because some of those attacked may have acquired partial immunity in the previous
attack.
As "trailer" succeeds "trailer" the cerebrospinal and pulmonary complications assume greater
prominence, the prevalences are more prolonged and tend to recur at longer intervals, and the striking
power of individual cases becomes less marked. After the lapse of six or eight years the characteristics
exhibited by influenza in its pandemic phase become greatly obscured, and save for slight annual increase
in the mortality from pneumonia, bronchitis, etc., the very existence of influenza in a town like London
may be almost entirely lost sight of. Gradually, however, protection of the population again wanes
and after a while anomalous illnesses, cerebrospinal fever, poliomyelitis, epidemic pneumonia, again
begin to attract attention; then later perhaps, lethargy and other protean manifestations of influenza
present themselves, and at length the moment arrives when the town suddenly realises that it is in the
grip of something which it feels can only adequately be described as a " new disease."
(c) Finally, the following further suggestions bearing upon the theory of influenza may be offered.
When cases newly introduced, say, in London some years after a pandemic prevalence, come
from a partially immunised area, they will have comparatively little infecting power, and the disease
they produce will be likely to differ widely from the typical influenza of pandemic phase; it may, indeed,
be even a cerebrospinal fever or poliomyelitis. Sooner or later, however, London will be exposed to
infection introduced from some possibly remote part of the world where influenzal infection is in a very
potent phase, and then (London being more apt to receive the infection and other conditions being
favourable) widespread influenza in pandemic phase may result. Under the conditions which have
obtained in the past, the United States has often been involved at a time when Europe was comparatively
free and vice versa. Of late, however, with improved means of communication, the great centres of
population have tended more and more to suffer together.
In the big prevalences of recent years the origin of the epidemic has often, as a matter of fact,
been traced to some sparsely inhabited part of the world. The phenomena may then be compared to
the waves in a pond in the middle of which a stone is thrown ; the waves travel outwards to the edges
of the pond and are then reflected back again. So, from the big centres of population the 'waves of
influenza travel outwards to remote parts of the earth and then, later, are in due course reflected back
once more. But the parallel must not be unduly pressed, for in the case of influenza the disease really
remains endemic in the thickly populated areas, though with steadily waning and then again growing
intensity; in the Athabascas and Kirghiz Steppes on the other hand it is not endemic; there are long
intervals of complete freedom, extending perhaps over one or two generations, it may be for a century
or more, but when the disease is at length introduced it almost at once gathers strength in so highly
susceptible a community, and it may acquire such power of spreading that it is reflected back again over
the civilised world.
It is submitted that these are some of the laws or principles which can be deduced from study of
influenza prevalences, and on some such lines as these the behaviour of influenza as an epidemic disease
can be explained without appeal to subterranean effluvia, earthquakes, comets or other phenomenal
causes.
W. H. Hamer,
Medical Officer of Health.
2, Savoy Hill,
Victoria Embankment,
London, W.C. 2.
June, ,1919.