PARASITIC ATTACK 



i. e., the optimum conditions are not necessary, although 

 they may be desirable, for the maintenance of either the 

 individual or the race. Protozoan parasites that are nat- 

 ural to a particular host are accordingly able to with- 

 stand considerable changes in temperature; are not af- 

 fected by the change from light to darkness or vice versa ; 

 and do not succumb to complex chemical changes nor to 

 increases in the density of the surrounding medium when 

 taken into that host; but why these same parasites are 

 not able to live in nearly related hosts is a problem of 

 great difficulty. A discussion of these questions is pre- 

 sented under the heading of Host-Parasite Specificity 

 (p. 42). 



6. THE parasite's METHOD OF ATTACK 



The character of the attack of the parasitic protozoon 

 on the host is of vital importance not only to the host but 

 also to the parasite. It is obvious that the degree of patho- 

 genicity depends on which organs or tissues are invaded, 

 and on the degree and rapidity of tissue destruction or 

 the production of toxic substances, since slight injuries 

 inflicted slowly are usually repaired by the host ; whereas 

 serious injuries that are quickly produced lead to symp- 

 toms. The association that exists in most cases of para- 

 sitism is such that the parasite is able to live and repro- 

 duce for many years within the host without apparent 

 injury to it. If the host develops severe symptoms both 

 it and the parasites are in danger and if the host dies the 

 parasites die with it. This type of parasitic attack is un- 

 usual and is considered to represent a comparatively re- 



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