_Spread of bubonic plague_.—With bubonic plague, unless it should in its course become pneumonic or septicaemic, there is almost solely the question of the rat and its fleas. Many authorities consider that pure bubonic plague can be treated safely in a general ward of a hospital provided there is sure freedom from bedbugs or other verminous insects. The various species of fleas which the rat may harbor may be attacked by the use of various petroleum preparations containing naphthalene. One preparation known as pesterine, which consists of kerosene 20 parts, soft soap 1 part and water 5 parts, the soap being dissolved in the water and the oil being gradually stirred into the hot mixture, is often recommended as a flea insecticide. A 5% solution of compound cresol to which naphthalene has been added is also of value. It is always well to combine flea destruction with rat extermination because, as rats are reduced in numbers, there are fewer hosts, so that man is more liable to infection with fleas deprived of a sufficient number of rat hosts. This may explain why a high death rate among rats, as the result of a plague epizootic, may act as a factor in the outbreak of human plague. _Rat destruction_.—As a matter of fact, however, it is best to attack the problem from the side of rat extermination which, it must be understood, is most difficult owing to the highly developed suspicious nature of the rat. A successful rat catcher or rat exterminator must have the mind of a detective. The first measure in rat extermination is the regulation of the disposal of garbage. It is most important that only cans with securely fitting tops be used so that rats cannot secure any food from the contents of the can. Again no particle of food should be left accessible to the rat. Unless the ordinary food supply of the rat is denied him he will not eat poisoned bait or bait in traps. Again rats are not only carnivorous but will eat any kind of cereal or vegetable, in fact they will eat almost anything and in addition are cannibals. In a plague outbreak especial attention should be directed to flooring in stables, under surfaces of board walks, sealed-in attics of houses, wharves and sewers. Where sewers have catch-basins at street openings the rat has a means of egress from the sewer. These sedimenting catch-basins also serve as a breeding place for mosquitoes. It has been estimated that a sewer rat can jump 2 feet but not 3 feet. In rat-proofing houses, double walls should be eliminated and houses raised well from the ground—at least 18 inches. In plugging up rat holes with concrete we should add broken glass to the concrete. Sheets of galvanized iron driven down several feet have been used as a protecting barrier around grain elevators or warehouses. Concrete is the proper material to use in rat proofing.