It is my opinion, as expressed above, that practically all of the population are rapidly infected during such a pandemic as we have had. The resistant have escaped, and it would appear to be very difficult to break down this resistance. The human experiment carried out by Pettenkofer on himself and his assistant with vibrion choleræ is an example, but we have numerous others demonstrating the same kind of phenomena in most of our diseases of established bacterial origin. In diphtheria we have an explanation in the varying antitoxic content of the sera, but we really know very little of what are the actual factors in preventing or determining infection among exposed individuals in the natural history of most diseases. The reports of Leonard Hill and Gregor are well worth reading in this connection, as well as the editorial in the same number of the British Medical Journal. We are not in a position to be very dogmatic on the causes of epidemics. The mere presence of the bacteria or any other living virus is not in itself sufficient to explain the phenomenon, and one of the chief objects of this paper is to indicate from the collected facts, that in the words of Flexner, “the case against the influenza bacillus is not proved.” _Conclusions_ 1. B. influenzæ is one of a group of hemophilic bacteria and there are probably strains of this organism which may be differentiated which will lead to further subdivisions of the group. 2. B. influenzæ as we understand it today, is distinguished by its morphological and staining characters; its requiring hemoglobin in some form for its development; its showing symbiotic reactions with other bacteria which stimulate its growth; the production of a toxine and its usual low pathogenicity for animals. 3. The media found most favorable for its growth are those containing blood with the hemoglobin content altered in certain ways, (1) by heating, (2) the addition of various chemicals, (3) by the action of other bacteria or their products. The heated blood agar I have found to be a most efficient and readily prepared medium. 4. Since B. influenzæ is so difficult to isolate, it is necessary to be very cautious in interpreting results unless the greatest effort has been made to demonstrate the presence of this organism. 5. B. influenzæ should be considered, from the evidence at hand, as the bacterial causative agent in epidemic influenza, and it should be recognized that secondary infections following the primary attack by this organism are both frequent and important. This view I believe the logical one, unless much more convincing evidence than we have today may demonstrate another more probable living virus as the cause.