out in the fæces; so that the flea serves not only as a carrier, but also as a multiplier of the germs.
Especially convincing are the experiments of the Indian Plague Commission,[1] which clearly show that if fleas are excluded healthy rats will not contract the disease, even if kept in intimate association with plague-infected rats. Young rats may even be suckled by their plague-stricken mothers and remain healthy. But if fleas are introduced, whether naturally or intentionally, into the field of experiment, plague at once begins to spread from rat to rat, and with a rapidity in proportion to the number of fleas present. The Commission has shown further that an epizootic of rat plague may start without contact or even proximity of healthy and 
Fig. 67.—Xenopsylla cheopis. infected animals. It suffices to transfer fleas from a plague animal on to a healthy animal, or to place the latter in a room in which plague rats have died recently and been subsequently removed. The fleas that have left the body of the dead rats, remaining in the room, convey the germs. The atmosphere of the room is not infective; for if the experimental animal be suspended in an open cage a few feet above the floor, it does not become infected. Nor is the animal infected if placed on the floor, if the precaution be taken to surround the cage with "tangle foot" so as to keep off the fleas. But if it be placed on the unguarded floor, either in its cage or allowed to run about, or even if it be suspended two inches above the floor—a distance not beyond the saltatory powers of the flea—it will become infected. The Commission obtained practically the same results in experiments with guineapigs in plague-infected houses. It is therefore no longer open to question that the important—probably the most important—agents in the development and perpetuation of plague epidemics are
- ↑ Journal of Hygiene, vol. vi., No. 4, April, 1906.
