38 
and decrease, and of numerical proportion between the two classes of 
patients, we would find a lower rate of infection among all patients 
who bad resided in tbe institution an intermediate length of time than 
among either all patients upon admission or all patients of longest 
residence. 
That this may obtain is shown by the following hypothetical case: 
Let it be assumed that the endemic rate of infection within the insti- 
tution is 15 per cent and that 100 patients enter the institution with 
25 per cent of infection and 100 other patients enter with 5 per cent 
of infection. The average rate of infection for the 200 patients upon 
admission would be 15 per cent, namely, the same percentage present 
among patients who. by reason of long residence in the institution, 
conformed in the amotmt of their parasitism to the endemic rate of 
the institution. Under the influences of the two elements above 
noted, namely. 1 the dying out of the infections present at the time 
of admission, because of removal from the previous source of infection, 
and 2 the implanting of new infections from the sources wit hin the 
institution, the infection among the 100 patients entering the hospital 
with 25 per cent of infection will gradually decrease until it reaches 15 
per cent, the amount which the conditions within the institution will 
support and propagate. Under these same influences the infection 
among the 100 patients with 5 per cent of infection upon admission 
will graduallv increase until it reaches the endemic rate, namelv. 15 
per cent. 
Let us make the further assumption that the decrease in the 
amotmt of infection among the patients highly infected upon admis- 
sion is such that at the end of five vears their helminthiasis has fallen 
mJ 
from 25 per cent to 12 per cent. and. on the other hand, that the 
infection among those with low helminthiasis upon admission has risen 
from 5 to 8 per cent. Expressed in actual figures instead of percent- 
ages. let it be assumed that in five years' time 20 of the 25 infected 
patients of the first class lose all of their parasites and that the same 
proportion namely. 4 cases of the second class lose all of their 
worms. 5Ve would then find 5 cases of the first and 1 case of the 
second class still showing original helminthiasis. The two classes 
would, however, be liable to the same number of new infections, say 
7. This would give us 12 cases of infection in the first class and 8 
cases in the second, a total of 20 infections 10 per cent h 
The average rate of infection, then, for the 200 persons after five 
years’ residence in the institution would be 10 per cent, namely. 5 
per cent lower than that among the 200 patients upon admission and 
even 5 per cent lower than the rate endemic within the hospital itself. 
It is possible to put this explanation to a rough test among our own 
cases. At the Government hospital we had a comparatively high 
frequency of intestinal worms among the incoming patients, due to 
