37 
In its last analysis the increase or decrease of intestinal helmin- 
thiasis among patients after admission to an institution and the rate 
of such increase or decrease are governed by the relation between the 
pertinacity with which infections present on admission cling to their 
hosts and the rapidity with which new infections are implanted from 
sources within the institution. For instance, if the infections present 
at the time of admission are very persistent and if the new sources of 
infection within the institution exert their influence promptly there 
would result a rise in the rate of infection. On the other hand, if the 
infections present at the time of admission disappeared quickly, before 
the sources of infection within the hospital had time to exert their 
influence by implanting new infections, the percentage of infection 
would fall. While the rate of infection might thus vary at different 
periods after admission, the general tendency would be toward one 
fixed rate, namely, the amount of infection in harmony with the con- 
ditions within the institution. In the case of patients entering the 
hospital with a relatively high frequency of intestinal worms there 
would eventually be a decrease in the percentage of infection among 
them; in the case of patients entering the hospital with a relatively 
low frequency of intestinal worms there would be eventually an 
increase in the rate. 
This explanation applies, of course, only to those parasites which 
do tend to die out upon removal from the original source of infection, 
and not to those, such as Oxyuris vermicularis , in which the infected 
person is able, by direct and continual autoinfection, to continue his 
infection indefinitely. In the case of such a parasite not only would 
we not necessarily expect the infections admitted to the hospital to dis- 
appear (in the absence of treatment) , but each case admitted would be 
a source of infection to other (uninfected) patients, so that the general 
tendency would be to an increase in the amount of infection as the 
institutional life of the patients lengthened. 
With the exception of Oxyuris vermicularis , and possibly also of 
Hymenolepis nana, all of the parasites which appeared among our 
cases must in the course of their life cycle pass through certain evolu- 
tionary changes after leaving the original host before 'they become 
infectious, and the time and the conditions demanded for this evolu- 
tion eliminate the possibility of direct autoinfection. In the absence 
of an external source of infection, therefore, these parasites naturally 
tend to disappear from the infected patients. 
If, then, some patients may experience a decrease and other patients 
an increase in the amount of helminthiasis after admission to an insti- 
tution, and, furthermore, if the rates of increase or decrease depend 
upon the relation between the longevity of an infection on the one 
hand and the implanting of new infections on the other, it follows that 
under certain conditions of initial rates of infection, rates of increase 
