PROBLEMS IN HOST-PARASITE SPECIFICITY 



have ever been reported with infections due to these 

 species. 



We may conclude therefore that the infective stages 

 of human protozoa frequently gain access to lower ani- 

 mals and that those of the latter gain access to man. The 

 entrance of the infective stages of a species of parasite 

 into a host is necessary for host-parasite specificity, but 

 is only one factor in this relationship. 



(3) What factors within a host enable nat- 

 ural PARASITES AND PREVENT FOREIGN PARASITES FROM 



BRINGING ABOUT AN INFECTION? To auswcr this ques- 

 tion we should consider that part of the host in which 

 the parasite lives as its particular habitat, just as we look 

 upon certain fresh-water ponds as the habitat of free- 

 living species. Both free-living and parasitic protozoa are 

 at times subjected to certain factors in their habitats that 

 are harmful, and successful life and reproduction de- 

 pend on the severity of these harmful factors. The diges- 

 tive juices of the host, for example, have been considered 

 destructive to trophozoites even of natural protozoan 

 parasites. The cyst wall of intestinal protozoa protects 

 the organism from many conditions outside of the body 

 and may play an important role in the initiation of an 

 infection ; for example, it may react to the digestive juices 

 of the host, or to secretions of the parasite within the 

 cyst stimulated by the intestinal environment, so as to 

 liberate the enclosed parasite and give it a chance to 

 maintain itself there; or it may fail to liberate the para- 

 site and thus prevent infection. We know so little about 

 excystation that nothing definite can be said on this 

 subject. 



49 



