338 MICROBIOLOGY OF WATER AND SEWAGE 



decompositions be normally produced there is necessarily a great 

 latitude and great adaptability. 



For this very reason a study of the individual organism and its 

 action upon specific materials throws no light upon the major 

 problem, which is, given fifty different types of organisms and fifty 

 different fermentable substances, in a mixture, what will be the course 

 of the reaction? Here the preferences, the adaptability and the antag- 

 onisms all come into play and while it is impossible to say what has 

 happened or how, it is readily conceived and, in fact, almost apparent, 

 that out of this heterogeneous mixture there will come a homogeneous 

 symbiotic family and an orderly sequence of chemical events, in 

 which metabolic needs and food supply are all delicately adjusted. 



PATHOGENIC BACTERIA. Prevalence and Longevity. Owing to its 

 origin and nature, sewage may at any time contain infectious material 

 and for the purposes of the sanitarian it is assumed that at all times the 

 germs of disease are present. Such an assumption is possibly in excess 

 of the actual facts and is only justified because it supplies the only pos- 

 sible hypothesis having an adequate margin of safety. The actual 

 prevalence of pathogenic bacteria obviously depends in the first instance 

 upon the amount of sickness in the contributing community. Further- 

 more, if, as we are coming to believe, a definite proportion of the popu- 

 lation are perpetual carriers of typhoid infection then to just as definite 

 an extent is the bacterial population of the sewage made up of typhoid 

 bacteria from apparently well persons. In addition to these, about 

 five one-hundredths of i per cent of the population of American cities 

 are suffering from the disease in acute form. Making due allowance 

 for the extra precautions that are, or should be taken in the care of 

 the dejecta, these persons constitute a definite and fairly constant 

 source of infection. 



In the case of the other infectious diseases of the alimentary tract, 

 and, possibly to a less extent in the case of tuberculosis, diphtheria, and 

 many others, these general statements are equally applicable, so that 

 the possibility of the occurrence of infectious material in sewage is 

 not a remote one, but definite and almost quantitatively determinable. 



As to the persistence of active pathogenic bacteria in the sewage for 

 any length of time the data are less exact. In the case of typhoid fever, 

 which has been more carefully studied than any other disease, the germs 

 are more persistent in pure water than in impure, but whether this 



