222 BACTERIA, YEASTS, AND MOLDS 



in Europe, Asia, and Africa, is known to be transmitted by 

 rats. Tuberculosis is sometimes transmitted by cattle, 

 through their milk, to children drinking it. There may 

 be other instances not so well known where larger animals 

 are the means of distributing infectious material. 



Water. The distribution of disease germs by means of 

 drinking water is chiefly confined to two diseases, typhoid 

 fever and Asiatic cholera. Typhoid fever is very common 

 and many epidemics are due to polluted drinking water. 

 The disease is caused by a well-known bacillus (Fig. 74), 

 and the method by which the water becomes contaminated 

 is very easy to understand. The bacilli live in the intes- 

 tines of the patient and are carried from him by the 

 excreta. This material may be thrown 

 upon the soil or into earth closets, and is 

 liable in either case to percolate through 

 the soil or be washed by rains into wells or 



^, .„ streams. Wells are filled with water that 

 Fig. 74. Bacillus 



of typhoid fever. ^^^ soaked through the soil, and are quite 

 readily contaminated with typhoid germs. 

 Hence well water has been a very common source of the 

 distribution of this disease. In most cities the excreta are 

 thrown into sewers and the sewage may empty later into a 

 river. Hence the drinking water of cities may sometimes 

 present very great danger. Cities frequently depend upon 

 the water of running streams, and nearly all streams of any 

 size in civilized communities are more or less contaminated 

 by sewage from houses or towns on their banks. Such 

 water will be likely occasionally to become infected with 

 typhoid bacilli ; so that rivers and streams are positive 

 sources of danger to communities that depend upon them 



