A Round-up of Undesirables 135 



from the rat. But the question is how? 

 Rats do not often bite men or closely consort 

 with them, nor do they notably contaminate 

 human food. But rats have fleas and when 

 these bite infected rats they borrow the 

 bacilli with the blood and if then they bite 

 a man the bacilli may be directly introduced 

 into his system. 



Much study has shown that, in fact, in 

 countries and regions where plague prevails 

 the rat and the flea are potent factors in 

 transmission. 



In California where plague has been lurking 

 for some years, some of the ground squirrels 

 have become infected with plague bacilli and 

 while this is not yet of threatening- import- 

 ance, it is disquieting that the infective agent 

 has not long since been completely destroyed. 



Smallpox and its Relatives 



It is very interesting that with all our study 

 of micro-organisms and long familiarity with 

 these diseases, smallpox, measles, and scar- 

 let fever should still remain riddles among the 

 common infections. These diseases are all 



