vn] FLEAS AND PLAGUE 95 



epidemic diseases was controlled and prevented by 

 knowledge. At San Francisco the fleas of man, rats, 

 mice, dogs, cats, ground-squirrels and gophers were 

 studied. It was found there, as elsewhere, that while 

 each species of flea has its particular host few are 

 unwilling or unable to attack man and other animals 

 when the host dies. 



There is good reason to believe that during the 

 last outbreak of plague in Manchuria the fleas carried 

 the bacillus from the marmots (Arctomys) to man. 



Plague can be transmitted by the human flea ; 

 but it may be doubted whether this often occurs 

 under natural conditions. The rat-fleas seem inclined 

 to take more readily to mankind than the human fleas 

 do to rats. Experiments at Bombay seemed to show 

 that, though the human flea was able to transmit the 

 plague infection, it does not transmit it as readily as 

 the oriental rat-flea. An explanation of this was 

 obtained when it was discovered that Pulex irritans 

 does not live well either on rats, or on guinea-pigs, 

 which were the subjects of the experiments. A count 

 of the fleas was made, each day, in a number of 

 experimental cages, in which live human fleas were 

 placed in company with wild Bombay rats. A great 

 number of human fleas were put into a flea-proof 

 cage along with a rat. Each day a census was taken 

 of the fleas still alive. After twenty-fours hours it 

 was found that little more than one per cent, of the 



