70 INFECTION AND IMMUNITY. 



mitted only by insects. Thus, for many years, yel- 

 low fever was held to be so contagious that not 

 only direct transfer from person to person was 

 admitted, but also through various indirect means 

 as by fomites. Typhus fever has often been cited 

 as the most contagious of all infections, yet mod- 

 ern studies point rather strongly to an exclusive 

 insect transmission (perhaps fleas or bedbugs). 

 The conditions in plague seem to be somewhat 

 more complex, in that both insect transmission 

 (fleas) and contagiousness prevail, the latter com- 

 ing into play in the pneumonic form of the dis- 

 ease. Dengue, which spreads like wild fire, pos- 

 sibly is transmitted only by the bites of certain 

 mosquitoes. 



In some instances the role of the insect is an ob- 

 ligate one i. e., transmission can occur in no other 

 way than by its bite. This is pre-eminently true 

 of malaria, which, virtually, is incapable of trans- 

 mission from person to person even when malarial 

 blood is injected into a healthy person. The par- 

 asite when it leaves the body becomes infective for 

 man again only after it has completed a sexual 

 development in the mosquito. In some instances 

 infection may be carried from person to person 

 or animal to animal by the injection of diseased 

 blood, yet under natural conditions the role of the 

 insect is an obligate one (yellow fever, sleeping 

 sickness, Eocky Mountain spotted fever). The 

 flea in plague transmission is an example of a 

 facultative role, since, as stated, this insect is not 

 the only natural means by which the disease is 

 conveyed from person to person. 



sources of Insects which carry and transmit infections nat- 

 infection. urally must have some source from which they 



