GENERAL REMARKS UPON INFECTION 5 



which are also contagious for evident reasons. The 

 fact that the infecting parasite must penetrate the 

 body of its living host is well illustrated by the in- 

 fectious skin disease known popularly as " itch " 

 (scabies). The itch insect deposits its ova and rears 

 its family in burrows beneath the epidermis, and thus 

 becomes the agent in the production of an infectious 

 skin disease. Certain other parasites, known as 

 pediculi, infest the surface of the body, especially in 

 localities covered with hair. As these do not pene- 

 trate the skin, their presence does not constitute an 

 infection. 



The fact that a disease may be transmitted through 

 a series of individuals either by contagion or in some 

 other way by inoculation, by contaminated drink- 

 ing-water, etc. is evidence that it is due to a living 

 disease germ of some kind and that it is consequently 

 infectious. An individual who has been stung by 

 a wasp or bitten by a rattlesnake is not infected but 

 poisoned. The symptoms resulting from such a bite 

 cannot be reproduced in another individual by inocu- 

 lation of blood or other material from the body of 

 the person bitten. But the bite of a rabid dog gives 

 rise to an infectious disease " hydrophobia "-which 

 may be transmitted by inoculation through a series of 

 men or dogs or other susceptible animals. 



The poison introduced by a wasp or rattlesnake 



