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The establishment of milk as the causative factor in an outbreak of 
typhoid fever is based on the following points : 
(a) A sudden and marked increase in the number of cases along 
the route of some dairyman, without a corresponding increase in the 
number of cases among persons living in the same sections of the city 
but supplied with milk from other sources. In a town supplied largely 
or entirely by one dairyman a sudden increase in the number of cases 
would not implicate the milk unless other facts pointed to it and 
other factors could be excluded, but in large cities, where the people 
of practically every square are supplied with milk by two or more 
dairymen, an increase in the number of cases distinctly on the route 
of a given dairyman is quite easily determined. This fact alone is 
evidence that the milk is responsible, and if an investigation reveals 
that at a time corresponding to the period on which the group of cases 
along the dairyman’s route must have been infected there was at the 
dairy or one of the dairy farms a patient with typhoid fever whose 
discharges could readily have reached the milk, the chain of evidence 
is sufficiently strong to justify the assumption that the outbreak was 
due to the milk supplied by this dairyman, especially if the cases can 
not positively be proven to have been due to some other factor. 
A study of chart No. 3, Bulletin No. 35, Hygienic Laboratory, 
United States Public Health and Marine-Hospital Service, shows 
strikingly the increase in the number of cases on the routes of the 
three dairymen in Washington whose milk supply during the summer 
and fall of 1906 was for a time infected. One dairyman (No. 4) had 
among his customers in June, 1 case ; in July, 13 ; in August, 6 ; in Sep- 
tember, 3, and from October 2 to 21, 32 cases. This sudden and 
marked increase in cases during October along the route of this dairy- 
man was not accompanied by any corresponding increase in the num- 
ber of cases among the people living in the sections of the city over 
which his route extended but obtaining milk from other dairymen. 
(b) The demonstration of the B. typhosus in the suspected milk. 
When this is done, the chain of evidence is, of course, complete. But 
frequently it can not be done, because in the period of usually three 
or four weeks — covering the incubation period, diagnosis, and report 
of the cases— elapsing from the time of infection of the cases and the 
recognition of the outbreak, the B. typhosus has disappeared from the 
milk. 
If cases of typhoid fever are not discovered to account for the 
infection of an implicated milk supply, it is well to examine bacteri- 
ologically the stools and urine of all persons who handle the milk at 
the farms and the dairy. In this way the source of the infection 
may be found in the discharges of some person who has the disease 
