34 
From the above figures it is seen that the lowest percentage of 
infection was present in the group of patients who had been in the 
hospital from. 4 to 8 years. The first group (less than lyear's residence) 
and the last group (over 15 years' residence) gave the highest and very 
similar rates of infection, each being between 12 and 13 infections 
per 100 persons. The frequency of infection in the second (1 to 3 
years' residence) and in the fourth (9 to 15 years' residence) groups 
differed each from the other by only about 1 infection per 100 persons 
and each of these groups presented a rate of infection intermediate 
between the first and third groups and the third and fifth groups, 
respectively. In other words, there was a progressive decrease in the 
rate of infection from the first , through the second, to the third group 
of patients : then a progressive increase from the third group, through 
the fourth, to the last group containing patients of longest hospital 
residence; the rates of infection in the first and last groups agree very 
closelv with each other: a similar agreement is seen between the 
second and fourth groups. 
These results would seem to indicate that the frequency of intes- 
tinal worms tends to decrease for a certain period of institutional life 
(6 or 7 years) and then to increase when residence in the institution 
is prolonged beyond that length of time. Before attempting an inter- 
pretation of our results under this heading, however, we will proceed 
to examine further our statistics concerning the prevalence of hel- 
minthiasis at different periods of hospital life by considering the fre- 
quency of each species of parasite in each of the above groups, and 
also by studying the results obtained among various classes of our 
patients. 
Following are the rates of infection at different periods of institu- 
tional life with each species of parasite represented: 
Examined. 
Infections. 
Per 100 
persons 
Trichuris trichiura: 
Under 1 year 
626 
58 
9.27 
1 to 3 years 
858 
66 
7.69 
4 to 8 years 
737 
32 
4.34 
9 to 15 years 
445 
29 
6.52 
Oyer 15 years 
585 
58 
9.91 
Oxyuris vermicularis: 
Under 1 year 
626 
1 
0.16 
1 to 3 years 
85S 
8 
0.93 
4 to 8 years 
737 
9 
1.22 
9 to 15 year 3 
445 
11 
2.47 
Oyer 15 years 
585 
13 
2.22 
Hookworms: 
Under 1 year 
626 
13 
2. OS 
1 to 3 years ' 
858 
12 
1.40 
4 to 8 years 
737 
5 
0.68 
9 to 15 years. 
445 
1 
0.22 
Oyer 15 years 
585 
0 
0.00 
