Parasitism and Disease 



ii 



the proper conditions for their rapid develop- 

 ment should at any time exist. While the size of 

 the parasite is always a factor in the damage that 

 it may do to the host the factor of numbers is 

 perhaps of still greater importance because of the 

 power of very rapid multiplication possessed by so 

 many of the smaller forms. 



Certain minute parasites in the blood may cause 

 little or no inconvenience, but should they begin 

 to multiply too rapidly some of the capillaries may 

 be filled up and trouble thus result. Or take some 

 of the larger forms. A few intestinal worms may 

 cause no appreciable effect on the host, but as soon 

 as their numbers increase serious conditions may 

 come about simply by the presence of the great 

 masses in the host even if they were not robbing it 

 of its nourishment. Many instances are known 

 where such worms have formed masses that com- 

 pletely clogged up the alimentary canal. Such in- 

 juries as these may be regarded as mechanical in- 

 juries. Some parasites injure the host only when 

 they are laying their eggs or reproducing the young. 

 These may clog up passages or some of them may 

 be carried to some more sensitive part of the body 

 where the damage is done. The guinea-worm of 

 southwestern Asia and of Africa lives in the body 

 of its host for nearly a year sometimes attaining 



