But while pure epidemiologic studies will not demonstrate the ultimate factor in the etiology, nevertheless these studies do subserve several most important functions. The bacteriologist, the immunologist, the serologist have accumulated a wealth of information since the 1918 pandemic, but as far as definite conclusions concerning the causative agent of the disease are concerned we are no nearer to the truth than we were at the time when Pfeiffer made his original observations. There is no incontrovertible evidence by which one may say that the influenza bacillus is or is not the cause of the disease. We must therefore await further studies and future discoveries. But we cannot await idly in the knowledge that new epidemics of the dread disease will surely come, probably mild ones in the next few years, and certainly severe ones again within a few decades. We must amass all of the available information concerning the mode of action of the disease, its manner of spread, its degree of infectivity, its distribution and the mode of its recurrences, and try to formulate from a study of the available facts some means of protecting ourselves against the epidemic, if not of preventing it entirely. In short, in the present state of the bacteriologic knowledge of the disease, we may say that the epidemiologic features are the only facts upon which we have to build in planning our defense. Today, the practical work in the eradication of influenza must depend chiefly, if not solely, on the general methods of preventive medicine. Many valuable monographs have been written on the subject, particularly following the pandemic of 1889–1893, but these have all emphasized features and phases of the disease which seemed at that time to be particularly important. Facts which seemed of extreme importance to the earlier writers are today in some instances considered relatively unimportant, while other phenomena which were but touched upon by the former investigators today have assumed deep significance. For this reason it is worth while to reproduce here the observations made in previous epidemics, and to correlate them with the facts developed in the abundant literature of the last few years, and to draw therefrom inferences as to the life and habits of the influenza virus, and conclusions as to the means of interrupting its progress. HISTORICAL.