Parasitism and Disease 



5 



change of conditions, become of prime importance 

 to-morrow. An animal that is well and strong may 

 harbor large numbers of parasites which are living 

 at the expense of some of the host's food or energy 

 or comfort, yet the loss is so small that it is not 

 noticed and the intruders, if they are thought of at 

 all, are classed as harmless. Or we may at times 

 even look upon them as beneficial in one way or 

 another. "A reasonable amount of fleas is good 

 for a dog. They keep him from brooding on being 

 a dog." 



But should these parasites for some reason or 

 other increase rapidly they might work great harm 

 to the host. Even David Harum would limit the 

 number of fleas on a dog. Or the animal might 

 become weakened from some cause so that the 

 drain on its resources made by the parasites, even 

 though they did not increase in numbers, would 

 materially affect it. 



Perhaps the most serious way in which para- 

 sites that are usually harmless may become of 

 great importance is illustrated by their introduc- 

 tion into new regions or, as is more often the case, 

 by the introduction of new hosts into the region 

 where the parasites are found. Under normal con- 

 ditions the animals of a given region are usually 

 immune to the parasites of the same region. That 



