138 ILLINOIS STATE ACADEMY OF SCIENCE 



tozoan diseases which are transferred from man to man 

 through the sole agency of certain species of mosquitoes. 

 Absence of these essential carriers of the disease pro- 

 ducing organisms precludes the possibility of either dis- 

 ease becoming established in any locality. Thus migra- 

 tion of a malarial patient into new territory would not 

 extend the range of malaria unless suitable species of 

 mosquitoes were present. 



In these facts may be found an explanation of the con- 

 dition wherein a host species is free from a given species 

 of animal parasite in part of its range though the species 

 may be a normal host of the parasite. In some instances, 

 however, a parasite has been known to utilize several 

 hosts and thereby in its range exceeds the range of dis- 

 tribution of any one species among its hosts. 



DIFFERENCES IN HABITS OF THE HOST 



Differences in habits, and especially in food habits, go 

 far toward explaining why some individuals of a given 

 species are infected by certain internal parasites while 

 others of the same species are relatively or entirely free 

 from infestation. Individuals of the same species may 

 show marked differences in food preferences. This is 

 just as true of other animals as it is of man. In the hu- 

 man species tapeworms of a given species are acquired 

 only through eating raw or rare beef. Obviously a strict 

 vegetarian could not acquire such a tapeworm. In like 

 manner trichinous infestation is a consequence of eating 

 pork infested with the larvae, so as a race orthodox Jews 

 are exempt from this parasite. 



In many animals the young are more heavily parasi- 

 tized than are the adults and in some instances the para- 

 sites characteristic of the parasitic fauna of the young 

 are lost in later life. This is usually correlated with 

 changes in food habits though in some cases it seems 

 probable that active immunity is acquired by the older 

 organisms. The omnivorous, non-sanitary habits of 

 children in part help to explain their susceptibility to 

 parasitic infestations, 



