THE DISTRIBUTION OF MICRO-ORGANISMS 179 



nation of the ground-water from unusual or abnormal conditions 

 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. 1 Bad odors and tastes in drinking water that is not 

 polluted with putrid material are usually due to minute green 

 plants (algae). 2 The diseases most commonly disseminated 

 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 usually die in two or three weeks. 

 Under exceptional circumstances these organisms may perhaps 

 maintain their vitality for a longer period. They appear, however, 

 to be less hardy than the ordinary water bacteria. As 

 we now understand these diseases, the organisms causing them 

 will be present only in a water-supply which has been recently 

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

 standing the rapid death of these organisms in water, they 

 may exist long enough to infect individuals habitually drinking 

 the water. Many epidemics of cholera and typhoid fever have 

 been traced to water polluted with the discharges from cases 

 of these diseases, and in a few instances the relation of the con- 

 taminated water supply to the epidemic has been established 

 beyond a reasonable doubt. 



By self -purification of water is meant the removal, through 

 natural processes, of contaminating organisms such as might 

 occur from the discharge of sewage into it. It depends upon 

 the sedimentation of the contaminating material in the form 

 of mud, upon the growth of the ordinary water-plants and protozoa, 



1 See Fuller and Johnson, "The Classification of Water Bacteria," Journal of 

 Experimental Medicine, Vol. IV, p. 609. 



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

 U. S. Department of Agriculture, 1902. 



