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Willis and Sydenham, they were taken to be the distinguishable constituent parts
of one and the same epidemic constitution." The last period, he says, "in which
epidemic agues were so recognised and named in England was from 1780 to 1785;
and in the midst of that also there occurred an epidemic catarrh—the 'influenza'
of the year 1782." Then he proceeds to the "influenzas or strange fevers from
1889 to 1893, in some respects the most remarkable in the whole history," which,
as he says, "would have seemed an equally composite group if they had fallen in
the 17th century and had been described in the terminology of the time and,
according to the then doctrines or nosological methods." As a matter of fact
the epidemics preceding and following the pandemic of 1889-90 were then described
as dengue, pneumonia (Middlesbrough, Scotter or other), poliomyelitis, cerebrospinal
meningitis and under various other designations.
And now there are the influenzas and epidemic agues of from, say, 1915 to 1924,
to be reckoned with. The "doctrines and nosological methods" of to-day point to
agreement, at any rate in this, that the epidemics classed as encephalitis lethargica
in 1917 and succeeding years (and notably the epidemics of 1921 and 1924) were
epidemics which, while they differ inter se in many remarkable respects, have common
characteristics which render it very difficult to assume they are not related to one
another. Those who have studied these epidemics most closely are more impressed
with the points of agreement than with those of difference. It is but a short step
further, following the synthetic method of Hippocrates and Sydenham, to accept
the conclusion that the whole group of epidemic diseases primarily affecting the
nervous system exhibits a close family likeness (epidemiologically speaking) and a
very close relationship, too, with influenza. The clinician inclines apparently to
agree up to a point and the bacteriologist, busily looking into the question of ultravisible
viruses, is becoming more and more disposed to regard with sympathy the
attempt to discover a field for the operations of these new proteges of his. Perhaps
no single phenomenon has been more conducive to this rapprochement than the
behaviour in the last eight years of encephalitis lethargica, for once the fact was
admitted that the prevalences of 1917, 1918, 1921 and 1924, despite the variations
in type displayed, were all outbreaks of the same epidemic disease, there could no
longer be any question as to the need for study of the remarkable relationship between
influenza itself and the associated outbreaks of pneumonia, bronchitis, cerebrospinal
fever, poliomyelitis and polioencephalitis, which have all assumed at one
time or another a role comparable to that played during recent years by encephalitis
.lethargica.
Again, recognition of the close relationship between these epidemic diseases affecting
the central nervous system facilitates acceptance as a working hypothesis of the
view that the "mutating ultravisible virus" of influenza is responsible for producing
not merely pandemic influenza but also the "trailers" which constitute the shadows
influenza casts before and behind. The marked changes which occur round about
the time of a pandemic prevalence, first from a less to a more, and then again from a
more to a less, pronounced power of spreading, suggest that some such change in the
ultravisible virus itself has occurred; and this view is confirmed by the altered
clinical manifestations observed in the trailers as compared with the pandemics.
Study of the
periodicity of
influenza in
its bearing
upon the
foregoing
hypothesis
relating to
the form of
the epidemic
wave.
The observations of Brownlee upon the periodicity of influenza are of special
interest here; indeed, the important question arises as to how far Brownlee's results
can be held to support the contention that accumulation of susceptibles is the main
factor at work; and as to how far they confirm the view that mutation of the ultravisible
virus, at any rate about the time of the pandemic prevalences, must be recognised
as a possibility.
The method of periodogram analysis employed by Brownlee leads him to conclude
that influenza and its trailers tend to recur during certain periods of years at intervals
which are generally speaking about 33 weeks or a multiple of 33 weeks ; after observing
this periodicity, however, for a few years "the rhythm breaks." Moreover, it must