486 INFECTION AND IMMUNITY. 



British East Africa near the source of the White 

 Nile. 



The opinion is held by many that plague is pri- 

 in squirrels! marily a disease of the rat and that certain regions 

 remain pest-infected because of this fact. Eats, in 

 certain districts, suffer from a chronic form of the 

 disease, and it is possible that the organism at 

 times acquires increased virulence, as a conse- 

 quence of which the infection becomes widespread 

 and rapidly fatal among these animals. It is 

 believed that transmission from rat to rat may 

 occur through the eating of plague cadavers. 

 Experiments are also reported showing that fleas 

 from plague-stricken rats will infect healthy rats, 

 guinea-pigs and monkeys by biting them. The 

 work of the Indian Plague Commission demon- 

 strated that the usual means of transmission from 

 rat to rat and from rat to man is by means of 

 fleas. Monkeys, which are readily infected if put 

 in the same room with infected rats, remain well 

 if protected from fleas. It has been repeatedly 

 demonstrated that fleas from rats and squirrels 

 will feed on man. 



In California, squirrels infected with plague are 

 an important source of infection in' man. Trans- 

 mission from rats to squirrels and from squirrels 

 to rats by means of fleas has been demonstrated by 

 McCoy, and McCoy and Wherry report a case of 

 transmission from squirrels to man, probably 

 through fleas. 



Flies, as well as fleas, may distribute the bacilli 

 from rats or the infected excretions of man 

 mechanically. 



When plague invades a new country it commonly 

 makes its first appearance in coast cities. Pre- 



