The province of Yunnan in China adjoins French Tonkin and British Burmah. It is of interest to the student of epidemiology because from this mountainous and difficultly accessible region there has issued but recently a disease which has been considered as practically extinct. Frightful as have been the ravages of the pest in the middle ages, it is noteworthy that during the past hundred years, with the exception of two slight outbreaks (Noja in Italy in 1815, and Vetlianka in Russia in 1878), the disease has been unknown in Europe. During this time the pest has not been extinct, but has existed to a greater or less extent in certain parts of Asia and in Africa. Four and possibly five of these endemic foci are known to-day. The province of Yunnan is one of these regions. The mountainous district of Gurhwal, lying along the southern slope of the Himalayas, is another center where the pest has continued to prevail. The recent travels of Koch in eastern Africa have brought to light a third region about Lake Victoria, in the British province of Uganda, and the German Kisiba, where the plague has existed from time immemorial, cut off as it were from the outer world. Only last year Sakharoff called attention to a fourth focus in northeastern China, and it is quite likely that a fifth focus exists in Arabia. These regions are of great importance in so far as the existence of permanent endemic foci sheds not a little light upon the development and spread of those great epidemics which, like great tidal waves, have in the past swept over whole countries and even continents. It is not known when or from whence the pest was first introduced into Yunnan. Unquestionably, it has existed in the extreme western parts of the province for many decades. Eventually the disease spread throughout the province, and frightful ravages are known to have occurred in 1871-73. Repeated visitations of this dread disease have taught the natives of Yunnan, as well as those of Gurhwal and of Uganda, to desert their villages as soon as an unusual mortality is found to prevail among the rats. In spite of the frequent recurrence of the plague, it did not spread to neighboring provinces, largely because of the fact that little or no communication exists between Yunnan and the adjoining Chinese states. Recently, however, the plague did succeed in crossing the frontier, and, in so doing, it has given rise to an epidemic which, as will be presently seen, has already made an unenviable record and has a future that no one can foretell.