CHAPTER V 



THE FLY: A STUDY OF DISEASE-PRODUCING 

 ANIMALS 



The great majority of human diseases are due to the multi- 

 plication in man's body of other organisms. Such organisms 

 are known as parasites. Parasites do not constitute a separate 

 group of organisms by themselves, like the butterflies, but 

 in almost every class certain species or even whole families 

 have come to live upon other organisms. The usual history 

 of the degradation of an animal to parasitism is as follows: 

 The ancestors of the animal have come to hve upon the waste 

 products of other animals or even upon the outside of other 

 animals. Later some of their descendants have come to 

 penetrate into the interior of the host, as the parasitized animal 

 is called, either through the skin or by way of the food-canal. 

 It is advantageous for them to do so, because in the interior thej^ 

 find not only better food, but warmth and protection as well. 

 The worst i^arasites are the smallest, because the small ones have 

 almost unlimited jDower of multiplication and because most 

 of them set free substances that act as poisons on the body of 

 the host. One of the most dangerous groups of these micro- 

 scopic parasites is that of the bacteria, which are usually classi- 

 fied as fungi ; that is to say, plants -without green coloring- 

 matter. Not all, but nearly all, of the bacteria are parasites. 

 Fully as dangerous, however, are the parasites belonging to the 



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