PLAGUE, RATS AND FLEAS. 261 



chiefly by ships or rail conveying merchandise. I have suggested that 

 infection of rats in neighbouring towns and villages is affected by means 

 of fleas carried by men. Finally, the breeding season of rats plays an 

 important part in the spread of plague in man. Plague, which is 

 essentially a rat disease, attacks men only when it is excessively pre- 

 valent among rats. The disease lingers on in these animals during the 

 off-plague season, and bursts out afresh among the rats when the 

 number of susceptible rats is increased by births ; and when the fleas, 

 the carriers and transmitters of the infection, are more plentiful. 



If plague, then, is essentially a disease of rats, are there any other 

 diseases which are peculiar to animals, and which are occasionally 

 communicated to man ? Does a study of these diseases furnish us with 

 any evidence which may explain by analogy how plague is, or is not, or 

 cannot be communicated to man ? How is plague communicated from 

 rats to men ? 



The following are epizootic diseases which are occasionally communi- 

 cated to man, and I would class plague along with them ; namely, 

 Anthrax, Glanders and Hydrophobia. 



Anthrax is a disease of cattle which is caused by a spore-bearing 

 bacillus. It gives rise to at least two forms of disease in man and in 

 this respect resembles plague ; namely, a disease called Malignant Pus- 

 tule which is produced by the inoculation of the bacillus under the skin ; 

 and " Wool Sorter's " disease, which is produced by the inhalation of 

 the bacillus, into the lung, as occurs in primary plague pneumonia. 



Glanders is a disease of the horse which is due to a bacillus, the 

 Bacillus mallei. The disease is generally found in man among farriers, 

 grooms, nakers and others who are associated with horses. In man the 

 disease is very fatal. 



You are all familiar with hydrophobia, which is a disease of dogs, and 

 which is occasionally communicated to man, and is in him a fatal 

 disease. 



Let me here digress by giving a popular exposition of the bacterio- 

 logy of these diseases. Germs or bacteria are divided into two classes, 

 the pathogenic, or disease-producing, germs ; and the non-pathogenic 

 germs, which are abundantly found, in nature, in the earth and air all 

 around. Now bacteria are fungi, which, as you are aware, are a class of 

 plants or vegetable organisms. In plant life, therefore, we find many laws 

 which are equally applicable to bacteria. Now, just as you know that 



