TYPHOID 357 



his infection from the preceding case, rather than from any general 

 route and that he is, therefore, a 'secondary case.' If he had 

 such associations, this is noted for further reference and the investi- 

 gator passes on to another bedside. If not, the questions continue, 

 and now at last take up milk, water, food, etc., but of course only 

 so far as to determine those used by the patient during his infec- 

 tion period. 



"Then the investigator passes to the next patient. What has 

 he learned so far? Nothing much yet. But he has narrowed the 

 possible routes of infection to certain water supplies, certain rnilk 

 supplies, certain food supplies, etc., L e., those used by the first 

 patient during a certain 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 inquiries 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 refer- 

 ence. If he be a primary, however, the origin 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 aliment- 

 ary 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, etc., must be one of those water supplies, milk supplies, 

 etc., used in 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 possi- 

 bility. But if the water supplies have been different, water is 

 eliminated from the question entirely. If the milk supplies are 

 identical, milk remains as a possible route of infection; if not, 

 milk is eliminated from the question entirely. 



In brief, provided the information obtained be reliable, and it 

 is part of the public health detective'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 established 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 30 patients. 

 The tabulation (probably already made in his own mind) shows, 

 say, 3 imported cases, 5 secondaries, 2 uncertain or indefinite. 

 The remaining primary cases show in common, say, 1 water supply 

 only, the milk, etc., varying; or 1 milk supply only, the water, 



