the difficulty of securing serum from overseas sources is greatly increased, so that we are well-nigh compelled to depend upon home-produced serum. In view of the uselessness of drug treatment it is plainly the duty of national, state and municipal authorities to keep on hand a reasonable supply of antipest serum to meet any outbreak. Manufacturers of biological products realize that the preparations for producing, storing and marketing antiplague serum are expensive and that the maintenance of immunized animals and the employment of expert serologists call for expenditures which are unlikely to be recovered from any demand for serum and that, moreover, the government is doing and will do all that lies within its power to make the serum unnecessary, by excluding plague. These are not encouraging conditions to lead American serum producers to add antiplague serum to the list of their products. If, under these adverse conditions, any producer of biologic products shall undertake to produce and maintain an adequate supply of antiplague serum, he will merit credit for a truly philanthropic service and will deserve the support of governments, national, state and municipal, as well as that of the medical profession. CHAPTER I ITS HISTORY AND ITS EXTENSION In plague there exists the most intimate relationship between cause and prevention. We will therefore set forth here, as briefly and concisely as their importance will permit, the principal facts related to the causation of the disease. Without an understanding of this relationship there can be no rational preventive treatment. These facts constitute one of the interesting stories of modern medicine: the story of the arrangement and interpretation of certain apparently unrelated facts, some of them long known to men, in the clear light of modern method; the story of the application of analysis, synthesis, logic and experiment, all leading to the creation of an understanding which permits us to battle successfully with _pestis bubonica_, one of the most ancient of human plagues. HISTORY.--This disease has an historic interest, most engaging and fascinating, which one finds it difficult to pass over with mere mention. I venture to recall, therefore, that plague almost certainly dates back to the pre-Christian era, the earlier record naturally being lacking in sufficient accuracy of description to enable us to identify the recorded epidemics, definitely and positively, with true bubonic plague.