PUBLIC HEALTH 



35 



occur in the I nitcd States, neitlier specifically associated 

 with plague except as previously outlined; other fleas may 



Fio 15. Tropical Rat-flea {Xenopnylla cheopia). 



act as carriers eciually well, but are not so abundant on rats 

 and do not bite persons so frequently. 



The plague bacilli (Bacillus pestis) ap])ear only in fleas 

 which have bitten affected persons or rats twelve to twenty- 



Fig. IG. Rat-flea of Temperate Regions 

 (Ceralophyllus faieiattu.) 



six hours previous to death, for after this time the bacilli do 

 not occur in the blood. The vitality and virulence of the 



