224 Microbes and You 



form of treatment— filtration or chlorination— is usually necessary 

 to insure the public a safe product. 



LAKES 

 Naturally occurring lakes provide water to some communities 

 and eliminate the necessity for constructing reservoirs. Since lakes 

 are also subject to pollution, the water must be treated before it 

 can be consumed with safety. 



RIVERS AND STREAMS 



Many large cities are located beside rivers, both large and 

 small, and such bodies of water may serve at least a two-fold 

 function. The upstream end may be used as a source of water for 

 the city, and sewage may be pumped back into the river at a down- 

 stream point. Major problems relative to river and stream pol- 

 lution have arisen through the years and represent some of the 

 knottier considerations facing public health authorities. Overload- 

 ing streams with sewage ruins the fish supply and has already 

 wiped out fishing areas that man used to rely upon for food. 

 Like reservoir and lake water, the supply pumped from rivers 

 must undergo some form of treatment before it is safe for human 

 use. 



There is a popular notion that running water purifies itself, but 

 in the light of newer knowledge this statement is not strictly 

 correct. Running water improves itself, but to say that the 

 water becomes purified is stretching the truth unless qualifications 

 are stated. The rate of flow, the degree of pollution, the tem- 

 perature, and other factors must all be considered before it is pos- 

 sible to state how far a body of water must travel before it can be 

 considered free of pathogenic organisms. We know that disease- 

 producing bacteria come out second best in competition with 

 saprophytic organisms in polluted water, but the time factor can- 

 not be stated without reservations. 



Sanitarians have made great strides in the last fift^^ years with 

 respect to improvement of water and milk supplies. Our morbidity 

 (sick rate) and mortality statistics reflect this change especially in 



