xx PUBLIC HEALTH AND PERSONAL HYGIENE 303 



are too careless in disposing of the sewage, and the 

 source of the supply of water becomes polluted by it. 

 In cities situated on a river which serves as a sewer 

 for one town and a source of water supply for others 

 farther down the stream, it often happens that a few 

 cases of typhoid fever, or other "water-borne" dis- 

 eases, in one town are the direct cause of epidemics 



A/P/ 



FlG. 161. Distribution of typhoid fever through pollution of a river. In- 

 tensity of epidemic shown by depth of shading. No. 2 flows into No. I ; 

 A-G, cities situated on the river; a water intakes. (Adapted from " School 

 Hygiene," issued by State Superintendent L. D. Harvey, of Wisconsin.) 



in cities located below. In some instances a single 

 case has been the cause of an epidemic. 



In the country, outbuildings and barns are frequently 

 so situated that the surface waters are drained into a 

 well near by. Most of the cases of " water-borne " dis- 

 eases occurring in the rural districts are due to polluted 

 water from bad wells. 



