THE STORY OF THE BACTERIA. I IQ 



under these conditions, determines an epidemic 

 of typhoid fever. 



The water supplies of large towns come, for 

 the most part, either from large rivers, or lakes, 

 or artificial reservoirs along the course of 

 smaller streams, or from artificial wells, which, 

 piercing the upper strata, gain access to the 

 deep underlying collections. Now in the sur- 

 face water supplies, as from rivers or from 

 lakes, man is his own worst enemy, because 

 the most serious dangers from impure waters 

 arise from its contamination with human waste. 



Many great water supplies, which, under 

 ordinary conditions are good, are constantly 

 liable to become sources of danger, because 

 the sewage from dwellings is discharged, if 

 not into them, still, so near to them that it 

 may now and then enter, being washed in by 

 rains or in some other way. This, under 

 ordinary conditions, may, if the sewage be 

 largely diluted in the reservoirs or streams, be 

 simply disgusting and filthy, though not posi- 

 tively dangerous. But if, as is at any time 

 liable to happen, typhoid-fever discharges get 

 into the waste-pipes and so into the water, the 



