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ing room for the population accruing from the purchased mice, a second 
breeding room not far away is maintained for mice bred at the Institute 
from perfectly healthy stock. This second breeding station has been 
developed from small beginnings and now has an average mouse population 
of 3000. The personnel in charge of each breeding room is distinct and 
does not mingle. During the period of many months in which the epi- 
demic continued, rose and fell in the one room, no death from mouse 
typhoid occurred in the other. 
Our attention has long been directed to the phenomena of the epidemic 
prevalence of disease, and since the early years of the founding of the 
Rockefeller Institute opportunity was presented to observe in succession 
epidemics of meningitis, poliomyelitis, influenza, and latterly of encepha- 
litis. The waves of these diseases, which have swept over the world 
since 1904, have aroused profound interest in the facts of epidemiology. 
We seized, therefore, upon this outbreak of mouse typhoid in order to 
study experimentally an epidemic disease among small laboratory animals 
which could be assembled and observed in fairly large numbers. The 
epidemic referred to has supplied the cultures of the Bacillus typhi 
murium for the experiment and certain data of a statistical nature with 
which to compare our experimental results. The healthy stock of mice 
also referred to provided an unexceptionable material with which to 
attempt the production of an intentional epidemic. 
The conditions of the experiment were simple. A kind of mouse vil- 
lage was set up and an isolated room away from all other animals was 
selected in which shelves were erected and the cages placed. The latter 
may be taken for streets and houses in an ordinary village. Each cage 
contained five healthy mice. The original population, later increased 
from time to time by the introduction of fresh increments, was 100. The 
epidemic was started by feeding with a virulent culture of Bacillus typhi 
murium ten mice placed in two cages midway of the other cages. The 
incidental contact between the culture fed mice and the others was secured 
by the attendant who fed the animals and periodically cleaned the cages. 
A spot map was kept in order to follow the events. 
The preliminary feeding experiments led to the death from mouse 
typhoid of eight of the ten hand-fed and seven contact mice distributed 
in as many cages. No epidemic in the real sense ensued. The fatal 
outbreak of mouse typhoid which arose partook of the nature of 
sporadic instances of the disease. 
When the conditions as regards death from the infection had become 
stationary (at about the end of 30 days), a fresh addition of 200 mice 
contained in cages of five each was brought into the village. The effect 
of this addition was striking: after a lapse of about five days the new 
