INSECTS AND DISEASE 



CHAPTER I 

 PARASITISM AND DISEASE 



PARASITES 



HE dictionary says that a parasite is a 

 living organism, either animal or plant, 

 that lives in or on some other organism 

 from which it derives its nourishment 

 for a whole or part of its existence. This defini- 

 tion will serve as well as any, as it seems to include 

 all the forms that might be classed as parasites. 

 As a general thing, however, we are accustomed 

 to think of a parasite as working more or less in- 

 jury to its host, or perhaps we had better say 

 that if it does not cause any irritation or ill ef- 

 fects its presence is not noted and we do not think 

 of it at all. 



As a matter of fact the number of parasitic 

 organisms that are actually detrimental to the wel- 

 fare of their hosts is comparatively small while the 

 number of forms both large and small that lead 



