ORIGIN OF EPIDEMICS 545 
matically, almost mechanically, he decides whether or not the patient 
has typhoid fever or not. Satisfied on that point, his first question is 
not, ‘ Tell me all the different water supplies you have used, or all the 
sources of milk you have used.’ The first question is, ‘ When did you 
first show the earliest symptoms of the disease?? Why? Because 
this date once fixed, the date at which infection entered the patient’s 
mouth is fixed also, i.e., a date between one and three weeks previous 
to the date of earliest symptoms.* Remember that at this stage the 
detective may not have even an inkling as to which of the usual factors, 
water, milk, food, flies or fingers, is involved. Still less can he guess 
which particular water supply, milk supply, etc., of the many possible 
ones, may be the guilty one. But the answer to this question reduces 
possible routes to those used by this patient—not at anytime—but during 
a specific period, i.e., from one to three weeks preceding his date of 
earliest symptoms. 
‘““ Not yet, however, are the milk and water questions offered. The 
second question is, ‘Where were you during that period?’ Why? 
Because if the patient were not in the community during that period, 
he could not have contracted his infection within it, and does not 
belong to the outbreak under examination at all but to some other.: 
He is, in brief, an ‘imported case’ and while, of course, he is to be 
supervised lest he spread his infection to others, he cannot help to 
locate the source of the main outbreak—wumless perchance he be himself 
that source, i.e., the introducer to the community of the original infection. 
If he be an imported case he is noted for further reference and the 
detective goes to another patient. If not, the questions continue. 
But not yet is water or milk or flies mentioned. The third question is 
‘Were you associated during your period of infection with any then 
known typhoid case?’ Why? Because such association, especially if 
intimate, makes it more than probable that the case under examination 
received 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 infection 
period. 
“Then the investigator passes to the next patient. What has he 
learned so far? Nothing much yet. But he has narrowed the possible 
* The occasional exceptions do not affect the validity of this statement as a 
practical working rule. 
