PRIMARY SITE OF INFECTION 



by way of the mouth in contaminated food or drink, or 

 by direct contact. The distribution of the protozoa after 

 they enter the body is at first due almost entirely to the 

 activities of the host and is determined by the point and 

 method of entrance. Intestinal-inhabiting species are 

 transported with the food through the esophagus and 

 stomach and into the intestine; species that are trans- 

 mitted by contact remain usually in the region of en- 

 trance, e. g., in the mouth or genital cavities. 



(2) Primary site of infection. Different species 

 of intestinal protozoan parasites become localized in dif- 

 ferent organs, tissues or cells of the body depending on 

 various factors. The digestive tract is more frequently 

 affected than others, a fact that is probably vitally as- 

 sociated with the necessity of the offspring to escape 

 from the host. The parasites may be ( i ) coelozoic, 2. e., 

 localized in cavities, such as the lumen of the digestive 

 tract, and genito-urinary cavities, or (2) histozoic, i. e., 

 within the tissues, where they may live among the cells 

 (intercellular), or within the cells (intracellular or cy- 

 tozoic). The factors that are responsible for localization 

 are not well known. The parasites are subjected to vari- 

 ous conditions in their environment just as free-living 

 protozoa are in theirs : they must protect themselves from 

 injurious agents, such as secretions and excretions ; must 

 reach a location where the proper nutriment is available ; 

 must possess some means of fixing themselves in a fa- 

 vorable situation ; and must be able to carry on reproduc- 

 tive processes. Certain intestinal flagellates of man, as 

 will be pointed out later, are of particular interest for 

 purposes of illustration. 



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