218 ELEMENTARY AGEICULTUEE 



look clear and pure and yet be dangerous to health. 

 The thing that most often pollutes water supplies 

 is sewage. Sewage is the term given to house refuse 

 such as grease, soap, and human waste. It has in 

 it millions of microbes or bacteria, some of which, 

 if taken in drinking water, cause typhoid fever, 

 others the cholera. Typhoid is one of the leading 

 causes of death in our country, and it is caused by 

 drinking water or milk polluted by sewage. 



River Water. A river which has passed many 

 towns in its course is not a safe water supply. Eivers 

 will purify themselves if the water is allowed to run 

 many miles without receiving a fresh supply of 

 sewage, but when they have been the receiving place 

 for the sewage of many cities the water becomes so 

 laden with deadly bacteria that epidemics of typhoid 

 have been known to break out in several towns 

 along the stream at the same time. 



Mosquitoes. Mosquitoes are troublesome and 

 dangerous pests which carry the germs of human 

 disease. Malaria is spread by mosquitoes, and in 

 no other way. The malaria microbes are taken into 

 the system of the mosquito with the blood sucked 

 from people troubled with disease. In the body of 

 the mosquito, the malaria microbe undergoes a 

 change that is necessary to its life, so if all mos- 

 quitoes were destroyed, malaria microbes could not 

 thrive and spread. "When the mosquito attacks a 

 well person, some of the microbes pass from it to 

 the human system as the insect sucks the blood. 



