BACTERIA IN WATER. 165 



teria in each c.c. Yet such milk is drunken with impunity, 

 and there is no reason for thinking that these great numbers of 

 bacteria are in the slightest degree injurious to health. Why 

 is it, then, that we look with indifference upon hundreds of 

 thousands of bacteria in milk and regard as suspicious water 

 which has them in simple thousands ? 



The bacteria present in milk come from sources which are 

 not likely to furnish pathogenic bacteria, and it is known 

 that ordinarily this vast number of milk bacteria is harm- 

 less. But in nearly all cases where water - is contaminated 

 with bacteria to the extent of several thousands per c.c., the 

 conclusion is that the water is ^itYz^r-contaminated. Unless 

 water is contaminated with either sewage or some source of 

 decomposing organic matter we do not find the number of 

 bacteria so high. A source of contamination sufficient to fur- 

 nish a body of water with this number of bacteria must be an 

 extensive one and in the majority of cases it is some form of 

 sewage. Now, of all types of bacteria, those found in sewage 

 are most likely to be injurious. When we remember that sew- 

 age contains every form of human excretions, and that it is 

 by means of the excretions that the pathogenic bacteria find 

 exit from the diseased patient, it will be easily understood 

 that sewage-contaminated water is likely to contain bacteria 

 which are pathogenic for man. Sewage-contaminated water is 

 therefore suspicious and dangerous, a fact abundantly proved 

 by the great prevalence of water-borne diseases in cities whose 

 water supply is contaminated with sewage, as in the cities 

 dependent upon river water for drinking purposes. When the 

 bacteria in water are in the thousands they render the water 

 unsafe, not because this number of bacteria is injurious, but 

 because such water is commonly sewage-contaminated. 



When we recognize the great chance which sewage-con- 

 taminated w r ater has of becoming impregnated with the germs 

 of human diseases, it is a little surprising to learn that the 



