nd
have been carefully studied and it has been definitely shown that many
of them are plague-infected. Just how the plague got started among them
will probably never be really known. There is little doubt, however, but
that it was transferred in some way from the rats to the squirrels. The
trains and the bay and river steamers running out from San Francisco
would afford abundant opportunity for the rats to go from the city to
the warehouses all along the shore. Once there they would use the same
runways as the squirrels about the warehouses and in the near-by fields.
In harvest time the rats migrate to the fields and make constant use of
the squirrel holes. The farmers in some sections report that they
frequently catch more rats than squirrels in traps set in squirrel holes
at that season of the year.

This close association of the rats and the squirrels affords a good
opportunity for the fleas infesting them to pass from one host to the
other.

So far only two species of fleas have been recorded from the
ground-squirrels. One, _Ceratophyllus acutus_, is very common, sometimes
literally swarming over the squirrels, particularly if a squirrel is
sick or weak from any cause. The other species, _Hoplopsyllus anomalus_,
is less abundant but still quite common. Both of these species infest
rats also, so the chain of evidence is practically complete. We have
only to assume that at sometime one or more of the plague-infected rats
found their way into the region where the squirrels were, and the fleas
passing from the rats to the squirrels would carry the plague with them.

The fact that the plague already has such a start among the squirrels
opens a new and very serious phase of the problem of suppressing the
disease. All who have hunted the ground-squirrels will testify to the
readiness with which the fleas from them will bite those who are
handling them. As it is the sick or weak squirrels that are most often
taken there is always a chance that plague may be transferred from them
to human beings. The records of the plague cases in California show at
least three cases in which there seems to be very little doubt that the
disease resulted from handling plague-infected squirrels.

[Illustration: FIG. 109--Human-flea (_P. irritans_); female.]

[Illustration: FIG. 110--Mouse-flea (_Ctenopsyllus musculi_); female.]

A still more serious thing is the possibility of the disease remaining
in a more or less virulent form among t