THE TRANSMISSION OF INFECTIOUS DISEASES 57 



be non-specific (merely mechanical) or specific. Thus, 

 when typhoid bacilli are carried by flies from some 

 infected feces to a pitcher of milk, the transmission is 

 merely a mechanical transfer of the bacilli on the fly's 

 legs and proboscis. It does not matter what species of 

 fly is concerned; in fact, it need not be a fly at all, 

 but some other insect. The transmission of plague 

 from infected rats to man is probably of this kind, and, 

 as far as our present knowledge goes, so is the trans- 

 mission of typhus fever by lice. 



Quite different are the circumstances governing the 

 specific transmission. Common examples of these are 

 malaria, yellow fever, and sleeping sickness. When 

 malaria is carried from one person to another by the 

 mosquito it is found that this is always a mosquito of 

 the species Anopheles. The ordinary Culex mosquito 

 is unable to effect the transmission. In malaria, as we 

 have already seen, the tiny parasite sucked up in the 

 patient's blood by the biting mosquito undergoes cer- 

 tain characteristic changes, and it is only after these 

 changes have been completed that the mosquito is able 

 to infect another person. But for some unknown reason 

 these changes occur only in the stomach of Anopheles 

 and not in other species of mosquitos. In yellow fever 

 and sleeping-sickness conditions are entirely analogous, 

 in the former a species of mosquito known as Aedes is 

 required, and in the latter' a species of biting fly know 

 as "tsetse fly" effects the transmission. According to 

 Rosenau poliomyelitis is transmitted in a specific non- 

 mechanical manner by the bite of the common stable 



fly. 



