74 INFECTION AND IMMUNITY. 



alive through inheritance in the tick during the 

 months of winter, and at other times through 

 alternation from tick to animal and animal to tick. 

 Man plays little or no role in its maintenance, and 

 his occasional infection through the tick bite may 

 be regarded, in a sense, as an unessential incident. 

 Yellow fever, again, illustrates the condition of 

 an acute infection in the animal host (man) and 

 a chronic in the insect (Stegomyia calopus). 

 Like Eocky Mountain spotted fever, the virus of 

 yellow fever is transmitted in a hereditary man- 

 lier to the next generation of mosquitoes (Mar- 

 choux and Simon d) in some instances at least. 6 

 Plague. The conditions in plague are peculiar in that the 

 disease runs an acute course in both the animal 

 hosts (rats and man) and in the insect trans- 

 mitter (flea). Eats do indeed suffer from chronic 

 plague in some instances, but this is not a septi- 

 cemic condition, hence it affords little or no oppor- 

 tunity for the infection of fleas. 



It may be questioned whether the presence of 

 plague bacilli in the stomach and intestines of the 

 flea constitutes a true infection, but it seems justi- 

 fiable to take this view of the condition, since the 

 bacilli apparently proliferate in this locality for a 

 few days at least (Verjbitski). Verjbitski found 

 that fleas will transmit plague for three days after 

 their infection, the Indian plague commission for 

 from eight to twenty-one days depending on the 

 temperature at which the insects had been kept 

 for twenty-one days at 75 -80 F., for eight days 



6. The British commission also appears to have been suc- 

 cessful in proving this hereditary transmission, although the 

 attempt had failed in the hands of Reed, Carrol and Agra- 

 monte, and of Rosenau and Goldberger. Possibly it occurs in 

 only a small percentage of the insects. 



