It is unknown in the tropics, it has prevailed most extensively in temperate climates and it is not of infrequent occurrence in cold countries like Russia, Sweden and Nor way. A moist and moderately warm climate is most favorable to its development, while intense heat or cold will usually arrest it.
Sanitary Factors. It abounds in places where the sanitary conditions are bad, some times in the lowlands, sometimes in the moun tains, wherever the drainage is poor, the air foul, the soil saturated with filth and the houses mean and overcrowded. Those who use proper hygienic precautions are not likely to get the disease, those who live amid good or relatively good sanitary surroundings have seldom been its victims.
Method of Conveyance. This is a germ disease and is caused by a specific organism known as the Bacillus pestis. It was discovered at Hongkong during the plague epidemic of 1894, by Kitasato and Yersin, working inde pendently of each other. It was found by them in the pus from the buboes, in the lymphatic glands, in the internal organs and in the blood. The bacillus enters the body in the majority of cases by inoculation through the skin, which usually implies that there is some abrasion or break in the skin through which the bacilli can effect an entrance. It has long been supposed that the disease was communicated by the medium of filthy soil, or filthy clothing, or rags, or discharges from the bodies of those who have had the disease. It is doubtless true that it may be conveyed by any or all of these means, if the germ is really there. But a more explicit statement would be that it is conveyed and distributed by the rat-flea Pulex cheopis. For some time it was supposed that the bacillus was also carried by bedbugs, but that is now questioned. The principal conclusions of the on the Plague Investigations in India' which was published in 1907 are the following: (1) Healthy rats contracted plague from in fected rats when the only apparent means of communication between the two was the rat flea. (2) In 21 experiments out of 38, 55 per cent of healthy rats living in flea-proof cages contracted plague, after fleas collected from rats dead or dying from septicamic plague were placed upon them. Hence the rat-flea can transmit plague from rat to rat. (3) Continu ous contact of plague infected rats with healthy ones does not result in the infection of the latter if fleas are excluded. (4) If fleas are present the disease appears and spreads in pro portion to the number of fleas. (5) Guinea pigs placed in plague infected houses become in fested with rat-fleas and often develop plague.
(6) Fleas taken from plague infected rats may infect rats in flea-proof cages. (7) Guinea pigs in plague infected houses do not get plague when protected from fleas. If in cages surrounded by sticky paper six inches in radius, they remain immune, as the fleas cannot jump over the paper. (8) Chronic plague may pre vail in rats. The rat-flea, therefore, is probably the only medium by which the plague is trans mitted and the present method is to destroy all rats in cities and villages, especially in sea ports, where the plague has appeared. Efforts have been directed not only at the destruction of rats ashore, but they have also been directed to preventing them from getting aboard ships.
Prevention. Preventive measures consist in destroying rats, and ground squirrels which also carry the germ-bearing flea, in examining all passengers and baggage from suspected ports, in destroying or disinfecting with live steam all suspicious clothing, bedding and mer chandise, and in applying an emulsion of kero sene oil to all furniture which is likely to harbor fleas. Human beings who are suspected to have been in contact with rat-fleas should receive intravenous injections of HaffIcine's prophylactic serum, which is the most reliable of any of the several serums which have been devised. This serum gives immunity for a few months and may abort the disease in its stage of incubation. The sputum, faces and urine of the sick must be disinfected and all abra sions of the skin must be constantly protected with collodion. The point of inoculation must be found if possible and cauterized with pure carbolic acid.
Treatment. Imperative are good nursing, the recumbent position and attention to the bladder which may become partially paralyzed. Fluid food should be given and plenty of water. Heart stimulants are indicated alcohol, digi talis, strophanthus, strychnia and adrenalin chloride. Good results were obtained at Hong kong from internal use of pure Mor phine and hyocine may be required to relieve pain. Treatment with serums is proving very satisfactory. Yersin's serum, a blood serum taken from horses that have been inoculated with the plague bacillus, has increased recover ies 15 per cent, and is an unquestioned preven tive. Venal injection has proved most effective. Consult Blue, Rupert, (The Post-Mortem Diag nosis of Plague) (Washington 1912) ; Creel, R. H., Plague' (ib. 1914); Guiteras, S. M., (Plague in Havana' (in Journal of the American Medical Association, Chicago, 2 Jan. 1915); Neufeld, F., and Seuchenbekampfung) (Berlin 1914).