PART V. MAN'S INTERRELATIONSHIP WITH 

 OTHER LIVING THINGS 



UNIT XV 



HOW DOES MAN CONTROL HIS ENVIRONMENT 

 FOR HEALTH? 



Preview. The body has been likened to an engine, in that each 

 requires fuel and oxygen to work, produces wastes, and must have 

 frequent rest in order to do efficient work. Both the machine and 

 body may eventually wear out. But we do not speak of a sick 

 machine, although we do speak of a sick person. What, then, is 

 health? It is evidently a state in which the human machine 

 runs efficiently. It is a state of well-being, or being well. A 

 person may so abuse his body through lack of sleep, or exercise, 

 or proper food that soon his body will not function properly. He 

 may poison his body with alcohol or nicotine, and injure some of 

 his internal organs so that he never recovers his former efficiency. 

 He may meet with an accident and be crippled, or he may be 

 attacked by some microscopic foes, bacteria, and suffer from 

 infectious diseases. Diseases caused by these microorganisms 

 cause more than half the common ailments of young people. 



We have already learned something concerning the relation 

 of bacteria and other colorless plants to disease. It is the purpose 

 of this unit to show how some animals play a part in the cause and 

 spread of disease. It is obvious that the relation is twofold. 

 Animals may be parasites in man, causing certain diseases, or 

 they may, acting as hosts, carry a parasite for part of their life 

 histories. The malarial parasite and the hookworm are examples 

 of the first type ; the mosquito, which carries the malarial parasite, 



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