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. EXPERIMENTAL STUDY OF HEALTH AND DISEASE