Vol. 7, 1921 
PATHOLOGY: FLEXNER AND A MOSS 
321 
mice began to die of mouse typhoid, and five to ten days later the old mice 
(original population) also began to succumb. The total losses in this out- 
break were 70 per cent, and the number of cages involved all, or 100 per 
cent. The epidemic endured about four weeks and then subsided, although 
an occasional death still occurred at intervals. 
When a second equilibrium had been established, another addition of 
normal or healthy stock was introduced into the village. The succession 
of events was about the same as that just described. However, the peak 
of the epidemic was less high, the total percentage of deaths being 5 
per cent, although 100 per cent of the cages was again attacked. After 
an interval, another equilibrium was reached, when still another addition 
of normal mice brought about a recrudescence, passing as in the other 
examples from the healthy new population to the exposed old population, 
with however a total percentage of deaths below the others. 
These fluctuations or movements of the epidemic consisting of a series 
of waves have been repeated now about ten times. The general character 
or type of the wave or curve is always the same, although the height or 
peak as determined by the percentage of mortality varies. In every 
instance the mortality rate among the old mice became finally about the 
same as among the new. 
The experiments are followed in two main ways: according to the 
mortality, controlled of course by bacteriological examination of the 
animals succumbing, and according to the living carriers of the Bacillus 
typhi murium produced. The feces and the urine of living mice are 
tested for the bacillus, and in general it may be stated that the percentage 
of carriers stands in inverse ratio to the mortality. In other words, 
high death rate means a low carrier rate and vice versa. An obvious, 
although by no means safe, deduction from this observation would be 
that the carriers have all recovered from non-lethal infection and hence 
are rendered resistant by an acquired immunity, and that the production 
of carriers is nature's way of limiting the epidemic and of bringing about 
the state of equilibrium described. 
The main results of the study presented may be viewed as the mere 
beginnings of an undertaking to follow artificially reproduced epidemics 
among animals, which not only should simulate the epidemics naturally 
or spontaneously arising, as we say, among men and animals, but 
which being controllable may be made to yield up some of the underly- 
ing conditions affecting the movements among those naturally occurring 
epidemic outbreaks which we represent graphically in the form of curves, 
but which in relation to cause and effect we still are so largely ignorant. 
The impulse to use epidemic diseases among laboratory animals to help 
solve the problem of epidemics among man has been felt by other experi- 
menters. In particular Topley 1 in London has independently made use 
