220 MICROBIOLOGY OF WATER AND SEWAGE. 



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 possible 

 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. Furthermore, if, 

 as we are coming to believe, a definite proportion of the population 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 suffer- 

 ing 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 general- 

 ity can be extended to sewage is debatable. Our best information leads 

 to the belief that any reduction in numbers of typhoid bacteria which may 

 take place within the sewer before discharge is of minor importance and 

 of slight sanitary significance. 



Discussion of other pathogens must be in even more general terms. 

 Information is almost wholly lacking and it can only be assumed for 

 purposes of safety that, in so far as organisms of these various types are 

 discharged into the sewer, they will persist to a certain extent in the sewage 

 until it is finally disposed of. If such disposal be by discharge into a 

 stream without purification, then the waters of that stream become pol- 

 luted with infectious material. Studies recently made by Sedgwick and 

 McNutt have indicated the possibility that many diseases, other than the 

 oft-quoted typhoid fever, may be transmitted in this way. 



