44 
TYPHOID FEVER IN DISTRICT OF COLUMBIA. 
estimate seems to be one based on the percentage of chances of con- 
tracting the infection out of the District for the number of cases under 
each of the above heads as follows: 
Cases. 
(a) 80 cases, chances 100 per cent 80 
(b) 32 cases, chances 75 per cent 24 
(c) 34 cases, chances 50 per cent 17 
(d) 32 cases, chances 25 per cent 8 
(e) 681 cases, chances 0 per cent 0 
(/) 7 cases, chances (?) per cent (?) 
866 Total 129 
On this estimate, and eliminating the seven cases about which no 
information regarding whereabouts during thirty days prior to onset 
of illness was obtained, 15.01 per cent of the cases investigated 
contracted the infection out of the District of Columbia. 
No attempt was made to make a detailed epidemiological investi- 
gation of the cases which contracted the infection out of the District. 
Therefore most of the following data apply to the 747 cases comprised 
under (c), (d), and (e) of the above classifications. 
PREVALENCE. 
In Table II, page 58, is shown the cases according to date of 
definite onset. As the cases studied were those reported from 
June 1 to October 31, inclusive, this table does not include all of the 
cases that actually occurred in May and in the latter half of October, as 
some of these were reported prior to June 1 or subsequent to October 
31. But all of the cases having a definite onset during the period 
extending from June 1 to about October 15 are embraced in the table. 
The actual time at which the infection was contracted could probably 
be more accurately calculated if the date of the occurrence of the first 
slight symptoms could be ascertained definitely for each patient, but 
after the interval of two or three weeks, which usually had elapsed 
between the onset of the disease and the investigation of the case, the 
memory of the patients, their relatives, or friends often was not clear 
as to the date on which the patient began to have feelings of lassitude, 
slight headache, or other mild symptoms, while the date on which the 
patient was taken with definite symptoms, such as a rigor, severe 
headache, etc., or on which he had to take to bed usually would be 
remembered with fair distinctness. In the investigation we endeav- 
ored to obtain both the date of occurrence of the very first symptoms 
and the date of definite symptoms, but as the statements regarding 
the former were in so many instances vague and unsatisfactory, the 
date of the onset of definite symptoms has been used in the tables, 
charts, etc., in this report. By going back about three weeks from 
the date of onset of definite symptoms as given, the date on which 
