154 HOUSEHOLD INSECTS 



"The vitality and virulence of the plague microbes are 

 preserved in these insects. 



"The numbers of plague microbes in the infected fleas 

 and bugs increase during the first few days. 



"The feces of infected fleas and bugs contain virulent 

 plague microbes as long as they persist in the alimentary 

 canal of these insects. 



"Infected fleas communicate the disease to healthy 

 animals for three days after infection. 



"The injury to the skin occasioned by the bite of bugs 

 and fleas offers a channel through which the plague 

 microbes can easily enter the body and occasion death 

 from plague. 



"Crushed infected bugs and fleas and their feces, like 

 other plague material, can infect through the small 

 punctures of the skin caused by the bites of bugs and fleas 

 but only for a short time after the infliction of these bites. 



"Human fleas do bite rats." 



The many investigations that have been made of the 

 relations of fleas to the bubonic plague by different plague 

 commissions and individuals point to the definite con- 

 clusion that rats and fleas, are, at least, the most im- 

 portant factors in the spread of the disease. The bubonic 

 plague is probably, primarily, a disease of rats and only 

 secondarily a human disease. Epidemics of the plague 

 seem to be usually preceded by this disease among the 

 rats of that locality. 



San Francisco, in her late fight with the plague, gave a 

 great deal of attention to controlling the rats, basing her 

 method of work upon the foregoing ideas of the relation of 

 rats to the disease and the relation of fleas to rats and to 

 man. The results of the fight were eminently successful 



