358 WATER AND DISEASE 



etc., varying; or no connection except attendance at some one 

 social function. 



"Going straight to the route thus indicated, the public health 

 detective quickly confirms the indications of his results. He knows 

 that the route indicated must be the guilty one, for only that 

 route can account for all the cases. He concentrates on that 

 route until the evidence is complete when and how that route 

 became infected, when and by what sub-routes the infection was 

 distributed, why it infected the patients found and not others, etc. 



"In this illustration I have assumed complete ignorance on the 

 part of the epidemiologist as to everything connected with the 

 community he is investigating, except what he finds by cross-exam- 

 ining the patients. As a matter of fact, every epidemiologist, 

 however much a stranger to the particular community he enters, 

 begins to learn about it from the moment he enters it. 



"Thus, almost unconsciously he notes the size of the town and 

 compares it with the number of cases reported as existing; if it 

 is summer time he almost automatically notes the presence or 

 absence of open toilets in the backyards, of manure piles and of 

 garbage cans all bearing upon fly infection. If it is winter time 

 or the community be well sewered, he does not even consider flies. 

 If the cases are grouped in one quarter of the town, while the public 

 water supply extends all over it, he tentatively eliminates the 

 water supply before he asks a question. If good surface drainage 

 and a sandy soil exist, or driven wells are chiefly in vogue, he 

 tentatively eliminates well water even before he registers at the 

 hotel. 



"This is not and cannot be a complete synopsis of all the com- 

 binations of circumstances which the epidemiologist meets. It is 

 intended to illustrate his methods and to show why they are incred- 

 ibly rapid and incredibly accurate how they eliminate specula- 

 tion and guarantee a correct solution which means, of course, the 

 achievement of the great end, the finding of proper measures for 

 suppression. 



"As soon as the route is indicated, he must go to that route, and 

 establish beyond peradventure that it was in truth responsible. 

 A water supply cannot convey typhoid if typhoid fever discharges 

 have not entered it. There is no object in attributing an outbreak 

 to fly infection from toilets into which typhoid feces have not 

 been discharged at such a time as to account for the cases. A 

 milk supply, not handled at some point by an infected person, nor 

 adulterated at some time with infected extraneous matter cannot 

 convey typhoid. Whatever his results, they cannot be true unless 

 they are consistent they should not be accepted unless they are 

 provable and proved. 



"If the public health detective is familiar with the communitv 



