PASSIVE RESISTANCE OF HOST 



cases of chronic neurasthenia and subnormal health ; the 

 evidence for this is as yet incomplete. It is obvious that 

 amoebic abscesses may occur in any part of the body pro- 

 vided local conditions are favorable. Just what factors 

 favor the localization and multiplication of the amoebae 

 in liver, lungs and brain and prevent infections in other 

 parts of the body are not clear. 



The details of the distribution and localization of para- 

 sites within the host are still to be determined, but it may 

 be said in general that distribution is due primarily to 

 the physiological activities of the host, and localization to 

 host-parasite interactions, the parasites setting up an 

 infection wherever favorable conditions exist. 



4. PASSIVE (natural) RESISTANCE OF THE HOST 



The obstacles that must be met by the parasite at the 

 beginning of an invasion constitute the passive, or nat- 

 ural, resistance of the host. This type of resistance is in 

 part due to the nature of the host without respect to an- 

 cestral relations with the parasite or may in part have 

 been built up during the course of evolution as a protec- 

 tion against infection. The changes in the environment 

 of an intestinal flagellate, for example, which is carried 

 from the outside into the digestive tract of a new host 

 are very striking (see p. 148). 



A parasitic protozoon may successfully withstand all 

 the conditions encountered in a new host but still fail to 

 bring about an infection because of the absence of fac- 

 tors necessary, for example, to weaken the wall of a 

 protozoan cyst and allow the organism within to escape. 

 The literature of protozoology contains many reports of 



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