HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



and set up new infections in order to maintain the race. 

 In most cases escape is easy since the parasites attack 

 parts of the host from which natural channels lead to 

 the outside, e.g., intestinal protozoa pass out with the 

 feces. The escape of sufficient trichomonads from the 

 mouth and vagina to keep these races from dying out is 

 probably brought about by kissing and coitus respec- 

 tively. 



III. Host-Parasite Specificity 



By host-parasite specificity is meant the character of 

 the relations between species of parasites and species of 

 hosts with respect to host susceptibility and parasite in- 

 f ectivity. Hosts and parasites may be divided into groups 

 and labeled according to their interspecific relations, 

 since observations and experiments have built up a con- 

 siderable body of facts regarding this subject; but what 

 environmental conditions and host and parasite charac- 

 teristics are responsible for the facts observed are still 

 very obscure. 



I. HOST SUSCEPTIBILITY 



Parasitologists have long recognized different types 

 of hosts with respect to their susceptibility to various 

 parasites. Thus if a host is easily parasitized by a cer- 

 tain species it is said to be tolerant, whereas if it is dif- 

 ficult to parasitize it is classed as refractory. A host that 

 is frequently found parasitized by a certain species in 

 nature is known as a natural or autochthonous host; 

 whereas one that does not become so parasitized may 

 be considered a foreign host. If a species of parasite that 



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