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prevalences is mainly determined by recurring accumulations of susceptible children;
waves follow one another with almost unfailing regularity, in London at intervals of
some eighteen months or two years. It was pointed out, however (Milroy Lectures,
1906), that the number of susceptible children in the population is by no means,
as might as first sight be supposed, nearly exhausted at the end of every measles
prevalence. The disease in fact is always smouldering, though it is only at (1½
or 2 yearly) intervals that it bursts into flame. There has been extraordinarily
little variation in the periodicity of London measles during the last 80 years; certain
small points of difference, which have been noted, are exhibited in diagram "D"
in the annual report of 1912, and as stated (loc. cit.) there seem to be reasons for
associating the phenomena with the greater measure of control over epidemics of
measles which has resulted since the coming into existence of the school medical
service.
Dr. Brownlee (Royal Society of Medicine, Epidem. Sec. 1918) suggested that
measles periodicity is influenced by the existence of varieties of measles germs, each
manifesting a cycle of activities in fixed periods, those of most common occurrence
being periods of 87, 97, 114, 109½, 105½ and 89½ weeks. The arguments against
this view, in so far as it might be held to imply variations in biological peculiarities
of the germs, were summarised in the annual report for 1919.
Influenza stands in most striking contrast to measles, the widely spreading
pandemic prevalences of the disease are preceded and followed in thickly populated
areas by trailing epidemics, which present characters sharply distinguishing them
from the pandemics, and furthermore, the trailers differ among themselves. The
early trailers following upon the pandemic prevalence are, as a rule, attended
by high mortality; in those next succeeding the mortality tends to diminish, almost
to complete extinction; later still the trailers again grow in severity, until at length,
twenty or thirty years from the last world-wide outbreak, there is again a pandemic
recurrence and following upon that a fresh cycle of events is entered upon.
The salient points to be noted in connection with the pandemic prevalences
are that there is rapid transference of infection from case to case; that the main
stress of the shock falls upon the nervous system; that the percentage of recoveries
is as a rule high; and that relapses and sequelae are not uncommon. In the trailers,
on the other hand, spread is less rapid, some one special system in the body
(pulmonary, gastro-intestinal or nervous) being particularly apt to suffer; casemortality
is greater than in the pandemic; and the illness in those who recover
may be protracted. The contrasts between the phenomena of the pandemic
and those of the neighbouring trailers are so striking that some authorities have
questioned whether the successive epidemics occurring round about a pandemic
prevalence are to be regarded as due to "influenza" at all. As Goodhart, however,
put it, no one will surely deny that the epidemic of 1891 was an epidemic of influenza
(i.e., an epidemic ejusdem generis with the great pandemic of 1889-1890). If 1891,
why not 1892, and so on? Where, then, can the line be drawn? Modern epidemiology,
thanks to the teaching of Sydenham, Willis and Huxham, and in later times to that
of Creighton and others, definitely affirms, at least, that influenza " casts a shadow
before and behind," and many hold that the shadows really cover the entire 20 to
30 year intervals separating one pandemic from another.
The fact that the pandemic prevalences stand in such marked contrast with
neighbouring trailer prevalences has been held of late years to imply that the germ
of influenza must be peculiarly subject to variation just before and after the pandemic
outbreaks. In the pandemic the infecting agent is presumably derived as a rule
from early cases of illness, it multiplies rapidly, produces toxins which affect the
central nervous system by "shock attack," and then leaves the individual sufferer
with fairly complete immunity, so far that is as liability to experience a second
"shock attack" is concerned. In the trailer, on the other hand, the infecting
agent is more deliberate, possibly prone to assume "resting forms," and apt to