DISTRIBUTION OF BACTERIA. 137 



her of organisms varies greatly in different places and 

 under different conditions. The number of different spe- 

 cies found in water is also very large. Ground-water 1 

 contains few or no bacteria under normal conditions, and 

 is therefore suitable for a source of water-supply, when a 

 sufficient amount is available. The possibility of contami- 

 nation of the ground- water from unusual or abnormal con- 

 ditions should always be eliminated before it is taken for 

 drinking-water. Numerous epidemics of typhoid fever have 

 been traced to contamination of wells. The location of wells 

 with reference to privy-vaults and other possible sources of 

 contamination should be chosen with the greatest care. 



The ordinary bacteria of water are harmless, as far as 

 is known. 2 Bad odors and tastes in drinking water that is 

 not polluted with putrid material are usually due to minute 

 green plants (algae). 3 The diseases most commonly dis- 

 seminated by water are typhoid fever and Asiatic cholera, 

 and probably also dysentery. The spirillum of cholera will 

 usually die in natural water (not sterilized water) inside 

 of two or three weeks ; the bacillus of typhoid fever will usu- 

 ally die in two or three weeks. Under exceptional circum- 

 stances these organisms may perhaps maintain their vitality 

 for a longer period. They appear, however, to be less 

 hardy than the ordinary water bacteria. As w r e now un- 

 derstand these diseases, the organisms causing them will 

 be present only in a water-supply which has been contami- 

 nated by the excreta from a case of the disease. Notwith- 

 standing the rapid death of these organisms in water, they 



1 Ground-water is the water which originally derived from rain or 

 snow sinks through superficial porous strata, like gravel, and collects 

 on some underlying, impervious bed of clay or rock. 



2 See Fuller and Johnson, " The Classification of Water Bacteria," 

 Journal of Experimental Medicine, Vol. IV., p. 609. 



1 " Contamination of Water Supplies by Algae." G. T. Moore in 

 Yearbook U. S. Department of Agriculture, 1902. 



