INFECTIOUS DISEASES. 293 



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 dis- 

 eases, but it is only during recent years that the fact of its transmis- 

 sion by personal contagion has been generally recognized by physi- 

 cians, 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 Eng- 

 land to be included in a tabular list of epidemics given by Hirsch. 

 From that time to the present very numerous epidemics have oc- 

 curred. Some of these have been limited to the eastern hemi- 

 sphere, 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 epi- 

 demics 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 pesti- 

 lence in Asia and Europe. Recent researches by the Japanese bac- 

 teriologist Kitasato and by the French bacteriologist Yersin have 



