OF THE ALIMENTARY TRACT 15 



man, and were those of the several divisions of man's 

 alimentary tract readily disseminated to other parts, 

 the case would be tremendously complicated and 

 much more important than it is. The popular con- 

 ception that we receive our protozoan parasites from 

 the animals with which we have surrounded our- 

 selves is generally not true. Consequently, the 

 matter of prevention comes to mean mainly the pro- 

 tection of man from contamination with the excreta 

 of his own kind. 



When, theoretically at least, we find a protozoon 

 producing a definite disease as a constant or common 

 result of its parasitism, it may be suspected that it 

 is not a natural parasite of this host and search for 

 its natural host should be made, since control of it 

 would probably involve control in the natural host. 



In naturally occurring infections there is ordi- 

 narily a state of balance between host and parasite. 

 Occasionally this balance does not occur or is broken 

 down. 



RESISTANCE AND IMMUNITY OF HOSTS 



Natural immunity, therefore, exists in most ani- 

 mals against the parasites of other species. This 

 may be broken down at times. There is also natural 

 immunity of some individuals to parasites of their 

 own species, acquired immunity. Definite knowl- 

 edge of the development of resistance to the parasites 

 under question here, following their invasion, does not 

 exist. It is indicated by Craig (1926), who reports 



