ESCAPE OF PARASITE FROM HOST 



of kala-azar and oriental sore. No therapeutic agents are 

 yet available for intestinal flagellates, ciliates, and coc- 

 cidia. 



Whether these drugs act directly on the parasite or 

 through the host is still in doubt. In a recent illuminat- 

 ing address Dale (1924) states the situation in the fol- 

 lowing words. 'The conception of a remedy not killing 

 the parasites immediately, but modifying their virulence, 

 or lowering their resistance to the body's natural de- 

 fences ; of a remedy not acting as such, but in virtue of 

 the formation from it in the body of some directly toxic 

 product, either by a modification of its structure or by 

 its union with some tissue constituent ; of an affinity of 

 the remedy for certain cells of the host's body, leading to 

 the formation of a depot from which, in long persistent, 

 never dangerous concentration, the curative substance 

 is slowly released; all these conceptions present them- 

 selves, again and again, as necessary for our present ra- 

 tionalisation of the effects observed. It can hardly be 

 doubted that they will potently influence the methods by 

 which, in the immediate future, new and still better spe- 

 cific remedies are sought. But though our practical aim, 

 in relation to the affinities of a remedy for the parasite 

 and for the host's tissues, may be radically changed the 

 meaning of these specific affinities, so delicately adjusted 

 to a precise molecular pattern, remains dark." 



II. ROUTE TAKEN BY PARASITES IN ESCAPING FROM 

 THE HOST 



As already noted, parasites must not only reproduce 

 within the host but their offspring must be able to escape 



41 



