THE CARRIER OF PLAGUE 227 



propagate these diseases. Similarly Avith plague, the 

 forces which have led to its extinction and curtailment 

 are those grouped under Sanitary Reform— improved 

 ways of living, less overcrowding, refuse removal 

 and destruction, sewering and water supplies, better 

 food, hospitals for the isolation of suspected cases, 

 strict sanitary control by means of medical officers and 

 sanitary inspectors ; these are the weapons of civilisation 

 which have steadily driven back the agencies at work 

 in transmitting plague. Now let us inquire what 

 these agencies are. Plague is a disease caused by a 

 bacillus, that is to say by a bacterial parasite or virus, 

 and belongs to the class of diseases which also em- 

 braces typhoid, cholera, tubercle, leprosy. I^ike all 

 the bacterial diseases, it may be transmitted in various 

 ways, such as by direct contact, by contaminated food 

 and clothing. But there is one special way by which 

 we know it is spread, and that is why we discuss it 

 here, namely by means of an insect, — in this case the 

 flea. It has been shown by several observers, notably 

 in 190G by the investigations of the Plague Com- 

 mission in India, that especially the rat flea, Pulex 

 chcopis, acts as the carrier. It appears that when the 

 flea takes up a meal of infected blood from a person 

 suffering from plague, the bacilli are not killed in 

 the body of the flea ; it has been shown that they 

 even multiply ; when the infected flea bites a rat or 

 a healthy human being it transmits the virus. The 

 rat flea, it would appear therefore, is the chief trans- 

 mitting agent, and this explains why the rat in common 

 with man is the greatest sufferer from plague 



