In this country no layman loses any sleep on account of bubonic
plague. Is that because it does not exist? Not at all. It comes to
our waters, even effects a landing sometimes. But we have a small
garrison of vigilant medical men on our coasts watching day and night
for that enemy, ready to give him instant combat if he comes. We sleep
in peace because we trust that garrison. Thirty years ago we did not
know what caused this terrible plague, but in 1894 the germ (_Bacillus
pestis bubonicae_) was discovered. Even then it was not known how the
disease was carried or what caused it to spread so rapidly――and before
it could be combated successfully, that must be known. A series of
experiments on living animals, chiefly rats, guinea-pigs and monkeys,
yielded the desired information and through these experiments we have
been delivered from this terrible scourge. It was known that rats were
subject to plague; consequently attempts were made to find out how
it was transmitted from one rat to another. The idea that it might be
carried by parasites occurred to several investigators. Accordingly,
healthy rats were placed in cages close to diseased rats; they remained
perfectly well until a few fleas were introduced. Then, almost
immediately, the hitherto healthy rats were stricken with plague.
Cages containing healthy monkeys were suspended over cages occupied
by diseased and flea-infested rats. At regular intervals the monkeys
were lowered nearer to the stricken rodents. The monkeys were all right
until they were brought within jumping distance of a flea, when they at
once contracted the plague. These and other experiments left no doubt
that rat fleas were the carriers among animals, and since rat fleas
also feed on man when their natural prey is not available, it was an
easy matter to show that the plague is spread by means of rat fleas.
This led to a definite program for checking the spread of the disease,
by relentless warfare on fleas and the rats that carried them. The rats
were trapped, their breeding places destroyed, and diseased rats from
infested ports were prevented from entering the country. For example,
when it was found that rats frequently come ashore along the cables
stretched between the ships and the wharves, metal cones similar to
those used to prevent rodents from climbing into corn cribs were placed
on the cables. The fact that I wish to emphasize is that it is due
to experiments on living mammals that this black death is no longer a
terror to us.