162 
one of the dairy farms a patient with typhoid fever whose discharges 
could readily have reached the milk, the chain of evidence is suffi- 
ciently 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. 
(b) The demonstration of the typhoid bacillus 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 between the time of infection of the cases 
and the recognition of the outbreak, the typhoid bacillus has disap- 
peared 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 in an 
ambulant and unrecognized form (temporary bacillus carrier) or of 
some one who has been carrying the infection for months or even 
years (chronic bacillus carrier). 
Besides the large groups of cases of typhoid fever caused by in- 
fected milk, there must be in large cities frequently single cases or 
small groups of cases which are due to infection in the milk and yet 
can not be traced to that source. In a community where factors other 
than milk were operating to cause a rather extensive prevalence of 
typhoid fever, 5 or 6 cases occurring within a few days among the 
customers of a dairyman supplying several hundred families with 
milk would direct some suspicion toward that milk supply, but if 
this small group of cases should not be followed by an unusually 
large number of cases on the route of this dairyman and no typhoid 
cases were found on the dairy farm or at the dairy, these 5 or 6 cases 
would be placed by the investigator among those due to causes unde- 
termined or to causes other than milk. In many such instances, how- 
ever, these groups of cases are doubtless due to infection introduced 
in one of the many possible ways — hands, clothes, flies, water for 
washing cans, etc. — into a part of the dairyman’s output of milk for 
perhaps only one day. 
In cities having milk supplied by a number of dairymen, if several 
of these small groups of cases among customers of different dairy- 
men occur at about the same time, a list of the farms supplying each 
of the suspected dairies should be studied, and if it is found that two 
or more of these dairies receive milk from any one farm, an investiga- 
tion should be made of that farm, and in this way the source of the 
infection for the several groups of cases may be determined. 
