THE observations to be recorded below are in many respects incomplete, and in view of the large number of papers which have already been written on the subject of influenza it is desirable to explain at the outset the reasons which seem to us to justify this publication. In the first place, the observations, though small in number, were made at various periods from the outset of the epidemic till 1920, and the same technique was employed throughout; whereas most publications on the subject have referred to investigations carried out during some shorter period or with varying technique. Secondly, some of the observations relate to cases seen at the very outset of the epidemic, and it is in regard to the incidence of Pfeiffer's bacillus in the early stages of the summer epidemic of 1918 that there appears to be most diversity of opinion. An extensive review of the bacteriology of influenza has recently been published by Fildes and McIntosh (1). These authors make out a strong case for Pfeiffer's bacillus as the cause of epidemic influenza. In developing this conclusion they summarize the published evidence for the existence of a filterpassing virus and show that it is decidedly weak. They also review the records of experimental evidence for the selective action of B. Pfeiffer toxin on the pulmonary tissues, which have rapidly accumulated in the last two years. There is, however, no detailed comparison in their review of the incidence of Pfeiffer's bacillus during the years preceding the epidemic with that in the epidemic period. The observations of the epidemiologists go to show that there is something like a ten-year cycle in epidemic influenza, a heavy incidence having been noted in 1869, 1879, 1890, and 1898, but not apparently between that period and the recent epidemic (Stallybrass (2)). If Pfeiffer's bacillus is the cause of influenza, it is to be expected that the investigations recorded with regard to its incidence should show a gradual diminution of infections due to that bacillus in the years following an epidemic, and a gradual increase in infections due to it in the years immediately preceding an epidemic. We have tried to get information on these points from the literature. Unfortunately the relatively small number of investigations recorded in which Pfeiffer's bacillus has been carefully sought in the absence of influenza epidemics, [Q.
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