VIABILITY IN WATER. 55 



higher up the stream, or by the discharges of even 

 a single patient. The throwing of typhoid dis- 

 charges on the bank of a stream has resulted in 

 severe epidemics. Eeservoirs may be infected sim- 

 ilarly. In some instances a city which derives its 

 water from an inland lake also empties its sewers 

 into the same body of water. Even when the sew- 

 age outlet is quite remote from the water intake, 

 surface currents, as caused by the wind, may carry 

 water, and hence infection, from the former to the 

 latter. In harbors the water may become infected 

 from the sewage of a ship which carries a case of 

 cholera. Streams have been contaminated by 

 washing in them the soiled linen of patients. When 

 excretions are thrown on the ground the micro- 

 organisms have been carried into wells by surface 

 water from which small epidemics have arisen. 



The very occurrence of water-borne epidemics viability 

 indicates that the micro-organisms concerned live j 

 for a longer or shorter period of time in ordinary - 

 waters. Various factors influence their longevity 

 in water, and their persistence at the point of first 

 contamination. Purer waters are not so favorable 

 for the life of the organisms of typhoid and chol- 

 era as those which contain a certain amount of or- 

 ganic matter and salts. On the other hand, an ex- 

 cess of organic matter when accompanied by many 

 saprophytic organisms also shortens the life of 

 these bacteria, particularly when in stagnant 

 water; and this principle is utilized for purifica- 

 tion of sewage in those systems which involve the 

 use of sewage tanks. A rapidly flowing stream 

 naturally results in purification more quickly than 

 one which is sluggish. 



