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becomes an epidemic. That is to say, such is the normal tendency of 

 this class of diseases, and I may instance cholera, typhoid fever, diph- 

 theria, influenza and la grippe as examples. There can be little doubt 

 that the [)lagues and pestilences, such as the Black Death which visited 

 England in the 14th century after having spread all over Europe and 

 caused the death of twenty five millions of peo^)le, were other instances 

 of zymotic diseases ; and the fact that such plagues and pestilences are 

 of so much less frequent occurrence now-a-days, and so much less malig. 

 naiit when they occur in the more civilized parts of the world than else- 

 where, gives us the first important clue to their comprehension, and we 

 may hope to their extermination. For it has been and is just in pro- 

 portion to the cleanly habits of a people that these diseases lose their 

 fatal character. This suggests a close connection between filth and dis- 

 ease, and the more carefully that we look into the matter, following this 

 clue, the more fully are we convinced that such is the case ; that clean- 

 liness of i^erson and surroundings is the first law of health. Still, this 

 does not fully explain the phenomenon of zymotic disease, since the 

 advent of a specific case of disease is necessary that the evil effects of 

 uncleanly habits may be fully emphasized. This would seem to imply 

 the existence of a specific disease virus or poison for each o' these epi- 

 demics, the spread of which poison was favoured by the prevalence of 

 uncleanly habits. Reasoning on this line led Pasteur, Koch, (John and 

 others to the discovery of what will hereafter be regarded as the most 

 important generalization of medical science in our century, namely, that 

 which points to the existence of a special microbe, bacillus or living 

 germ for each of the so-called zymotic diseases. The next step was to 

 attempt the isolation of this germ, and with certain diseases this has 

 been done. In the case of Anthrax, Koch has cultivated the bacillus 

 and studied it throughout its complete development. The chart before 

 you will serve to give an idea of the appearance of this enemy of man- 

 kind, as magnified about 15,000 times linear. In the next diagram I 

 shew you both in situ, and isolated the bacillus tuberculosis from photo- 

 graphs by Koch. It is the study of this bacillus which has made Koch's 

 name so widely known within the last year ; but my purpose in empha- 

 sizing the matter is to draw your attention to the explanation which this 

 theory of zymotic diseases offers of their sudden spread. The specific 



