546 EPIDEMIOLOGY 
routes of infection to certain water supplies, certain milk supplies, 
certain food supplies, ete., 1.e., those used by the first patient during a 
certaan period, and he has done this in thirty minutes—in scarcely the 
time it takes for the old-style investigator to get his bottles ready to 
collect his first water sample. 
“‘ At the bedside of the second patient, the same inquities in the 
same order are made. If this second patient be an imported case, or a 
secondary case, he also is merely noted for future reference. If he be 
a primary, however, the origins of his drinking water, milk, food, etc., 
during his infection period are also ascertained. Perhaps he coincides 
with the first patient in every detail of alimentary supplies, in history 
and associations. If so, nothing much has been added to the detective’s 
knowledge. But more than likely, dissimilarities have developed. 
Since the responsible water supply, milk supply, ete., must be one of 
those water supplies, milk supplies, etc., used an common by primary cases 
all those not common to both of these primary cases may be dropped 
from consideration (except in rare instances of multiple routes). Thus, 
if both have used the same water, water from that origin remains as a 
possibility. But if the water supplies have been different, water 7s 
eliminated from the question entirely. If the milk supplies are identical, 
milk remains as a possible route of infection; if not, milk ts eliminated 
from the question enitrely. 
“In brief, provided the information obtained be reliable, and it is a 
part of the public health detcctive’s training to distinguish at a glance 
truth from falsehood, the honestly mistaken, or forgetful, or stupid 
replies from the reliable ones—and above all never to believe anything 
(to the extent of recording it) unless it is checked, confirmed and estab- 
lished as a fact, the modern investigator has in one hour narrowed his 
investigation to a point which the old-style investigator often would 
not reach for weeks. 
“And so from patient to patient the inquiry proceeds. In the course 
of the day the investigator has seen perhaps thirty patients. The 
tabulation (probably already made in his own mind) shows, say, three 
imported cases, five secondaries, two uncertain or indefinite. The 
remaining primary cases show in common, say, one water supply only, 
the milk, etc., varying; or one milk supply only, the water, ete., vary- 
ing; 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 
