CHAPTER XVII 



WATER AND ICE AS SOURCES OF INFECTION 



Contaminated Water 



WE have seen in another part of this 

 book that natural surface waters al- 

 ways contain considerable numbers of living 

 bacteria of various kinds, which are growing 

 and proliferating there, and no doubt actually 

 purifying the water in a certain way by feeding 

 upon and removing from it organic material 

 which has collected or been dissolved. Now 

 these bacteria in moderate numbers, under 

 ordinary conditions, are not at all harmful to 

 the consumer of the water for drinking or 

 culinary purposes. 



But, on the other hand, if the water be- 

 comes stagnant and the ordinarily harmless 



bacteria collect in very large numbers, it has 



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