CHAPTER X. 

 PARASITISM AND PATHOGENESIS. 



The Parasitic Relation. The presence in a living organism 

 of one or several organisms of another species, which live as para- 

 sites upon the first, is a phenomenon of common occurrence in na- 

 ture. Those organisms such as the bacteria, which are too small 

 to harbor visible internal parasites, are subject to the parasitic 

 ravages of larger beings such as amebae and other protozoa, 

 which engulf them bodily and digest them. Man, who is wont 

 to complain of his parasitic ailments, takes all his protein, fat 

 and carbohydrate from the bodies of plants and other animals. 

 Parasitism in the larger sense is a well-nigh universal character- 

 istic of living beings. Parasitism in a narrower sense usually 

 applies to the existence of a smaller organism, the parasite, in 

 or on the body of a larger, the host, a relation in which the host 

 furnishes the parasite its necessary food. In many instances the 

 advantages of the relation are wholly one-sided, but in others 

 the two organisms seem to be of mutual benefit. In the latter 

 case, the condition is called symbiosis. The infection of the 

 roots of the clover with Pseudomonas radicicola, which promotes 

 the nitrogenous nutrition of the plant, is an example of this rela- 

 tion. In other instances the two organisms living in close associa- 

 tion seem neither to help nor injure each other. They are then 

 called commensals or companions at the same table. Internal 

 parasites occur in all the higher animals and plants, and have 

 been found even in the bodies of protozoa. Representatives of 

 all the great classes of micro-organisms are found among the 

 internal parasites, and many more highly organized animals 

 and plants also lead parasitic lives. Man, alone, is subject to 



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