14 PROTOZOAN PARASITISM 



on or within another living organism" is a parasite. 

 There is something to be gained by adhering to this 

 conception and, following Craig's example, I have 

 taken this definition from Fantham, Stephens and 

 Theobald (1916). In this sense all the organisms 

 here considered are parasites of man and are so 

 classed. Some of these parasites are pathogenic and 

 some are apparently not. 



As has been frequently called to attention the 

 natural host of a parasite is one which has usually 

 become adapted to it and which is ordinarily done 

 comparatively little harm by it. Otherwise the para- 

 site would have but little chance of survival, since 

 it is dependent upon the opportunity of transmission 

 to other hosts before its own death occurs with the 

 death of its carrier. Consequently when severe dam- 

 age is done the host by the parasite it is to be suspected 

 that this is an unnatural association. 



SPECIFICITY OF PARASITE FOR HOST 



Further, it has come to general recognition that 

 there is more or less rigid relationship between para- 

 site and host, even to the point that most of them are 

 able to survive only in their own particular hosts and 

 in only one particular place in that host. These are 

 very important points in considering the relation of 

 protozoan parasitism in general to human medicine. 

 Were this not the case, were the very great number of 

 protozoa parasitic in other animals transmissible to 



