IX SECT TRANSMISSION. 69 



of an epidemic will not be ripe until a new gen- 

 eration of rats has been bred. The immune 

 rats do not harbor plague bacilli, and hence 

 cannot infect fleas. Kocky Mountain spotted 

 fever prevails only in the months of spring. At 

 this time the tick which acts as transmitter is in 

 its adult stage and readily feeds on man as well as 

 on other animals. The larval and nymphal stages 

 appear at other seasons, and, although their bites 

 are infective, they rarely feed on man, either from 

 lack of opportunity or because of a preference for 

 other hosts during these stages. The observation 

 of Carter, that when yellow fever patients are first 

 imported into a new district a definite period 

 (two to three weeks) elapses before new cases de- 

 velop, suggested some novel mode of transmission, 

 which eventually was proved true when Beed, Car- 

 rol and Agramonte proved by experimentation the 

 correctness of the mosquito theory of Carlos Fin- 

 lay and worked out the details of transmission. 



For some time it was supposed that insects 

 transmit only those diseases which are caused by 

 protozoan organisms. This impression arose from 

 the fact that the first examples of definitely proved 

 insect transmission concerned protozoan diseases, 

 as Texas fever (a piroplasmosis) of cattle, malaria 

 of man and birds, and more recently the try- 

 panosomatic diseases. It is only since the rela- 

 tionship of the flea to plague, of the tick to the 

 South African tick fever of man, and of other 

 mites to spirilloses of animals that the importance 

 of insects in transmitting bacterial diseases has 

 been recognized. S2? a fim5-~ 



It is an interesting fact that contagiousness is lated by in- 



, . , 5 . ,. i i sect Trans- 



sometimes simulated in a disease which is trans- 



