PUBLIC HEALTH 



35 



occur in the United States, neither specifically associated 

 with plague except as previously outlined; other fleas may 



FIG. 15. Tropical Rat-flea (Xenopsylla cheopis). 



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

 and do not bite persons so frequently. 



The plague bacilli (Bacillus pestis) appear only in fleas 

 which have bitten affected persons or rats twelve to twenty- 



FIG. 16. Rat-flea of Temperate Regions 

 (Ceratophyttus fasciatus.) 



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

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



