124 THE PLAGUE 



Infected rats are kept in one side of a cage, uninfected ones in the 

 other side. So long as the two are separated by a flea-proof partition 

 the infection is not transmitted. Even when the partition is made of 

 tangle-foot which catches the flea when it tries to pass to the other 

 side, the sound rat remains sound. When such a partition is removed, 

 the sound rat becomes infected. Rat fleas are captured and fed on 

 cultures of the plague bacillus and then freed in a cage occupied by 

 healthy rats which soon become infected. Infected rats are stripped 

 of all their fleas and then placed in cages with healthy ones, which 

 remain uninfected. Infected rats carrying their fleas are caged with 

 guinea-pigs, and the latter become infected. 



These with many other variations on the experiment have been 

 made, and show conclusively that the plague is transferred from rat 

 to rat or from rat to guinea-pig by the bite of the rat flea. This 

 question is satisfactorily answered. 



How is the plague transferred from rat to man? Evidently 

 experiment along this line might be open to serious criticism and we 

 must depend on observation, which is never so thoroughly convincing 

 as experiment. Will the rat flea bite man and if it does will such a 

 flea bearing the plague bacillus infect man? While we cannot use 

 man in this experiment, we may employ his nearest relative, the ape. 

 The rat flea does bite the ape, and if the flea is infected with the 

 plague bacillus its bite induces this disease in the ape. It is the testi- 

 mony of several observers that uninfected rat fleas, while preferring 

 their own host, will in its absence feed on man. 



Liston reports that in a certain house in India on April 6, 1904, 

 many rats were found dead; on April 11 the house was so infested 

 with fleas that the occupants sought sleep on the veranda. On April 

 17, two of these people were stricken with the plague. On April 20, 

 thirty fleas were captured in this house and of these fourteen were 

 rat fleas, while of 246 fleas captured in uninfected houses, not one was 

 a rat flea. Tidswell of Sydney states that on a wharf where many 

 rats were dead he was violently attacked by fleas. 



It has happened that in some localities rats die of the plague, while 

 there is no infection among men. This is explained by the absence 

 of the flea, which, while widely distributed, is not found on all rats 

 at all times. However, there is another factor which needs considera- 



