causation of these diseases. This will become still more apparent when we come to speak of the geographic range of infectious diseases in which there is an external development of the specific infectious agent, for such development is strictly limited by conditions relating to climate, soil, elevation above the sea level, etc. Thus yellow fever, cholera, and the malarial fevers are essentially diseases of warm countries, or of the summer season in those portions of the temperate zone in which they prevail.
Having thus called attention in a general way to the factors which influence the geographic distribution of infectious diseases, I shall now ask your attention to a brief account of some of the more important of these diseases considered separately, and in doing so it will be necessary to refer also to their geographic distribution in past times, or, in other words, to the history of epidemics.
Epidemic influenza, or as the French call it la grippe, is a disease which has frequently prevailed in all parts of the civilized world, and can not be said to have any definite geographic habitat. In this regard it corresponds with smallpox and other contagious diseases, but it is only during recent years that the fact of its transmission by personal contagion has been generally recognized by physicians, and indeed it is still denied by some. This fact, however, I consider to be well established. While references to this disease are found at a much earlier period, it was not until the year 1173 that it was described with sufficient accuracy by medical writers to justify the epidemic of that year in Italy, Germany, and England to be included in a tabular list of epidemics given by Hirsch. From that time to the present very numerous epidemics have occurred. Some of these have been limited to the eastern hemisphere, or to a restricted portion of it, while others have extended to the western hemisphere and have gained a wide prevalence on this side of the Atlantic, notably so the recent prolonged epidemic which dates from 1889. If we look at a list of the recorded epidemics during the present century we shall find that the disease has probably never been entirely absent from some portion of the eastern hemisphere, although it has been comparatively restricted in its range at times, and has again gained a wide extension in Europe and Asia, and has on numerous occasions crossed the Atlantic and invaded the western hemisphere. This occurred in 1807, 1815, 1824, 1830, 1832, 1843, 1848-'51, 1857, and in 1873-'75.
Bubonic plague is a fatal infectious disease which prevails at the present day in certain portions of China and other Oriental countries, and which in the past has prevailed as a devastating pestilence in Asia and Europe. Recent researches by the Japanese bacteriologist Kitasato and by the French bacteriologist Yersin have