An epidemic of the second century B.C., as described, seems to have been one of true plague, while the pandemic which began in Egypt in the sixth century A.D., thence extending to Constantinople, Europe and the British Isles, was certainly the disease known in modern times as the plague. This pandemic, beginning as the plague of Justinian, was probably followed by the continuous presence of the disease in Europe, marked by many local outbreaks and periods of quiescence and extending down through the centuries to the period of the Crusades. In the eleventh and twelfth centuries the returning Crusaders spread the plague widely through Europe, which country it ravished from the eleventh to the fourteenth centuries, reaching its climax of intensity in the "Black Death" of Europe of the Middle Ages. The disease thereafter continued to devastate Europe, the great population centres, Paris and London, suffering especially from its visitations and its more or less constant presence. The Great Plague of London, the last important epidemic of the disease in that metropolis, began in 1664 and lasted five years. With less than half a million of inhabitants it is estimated that London gave one of every six or seven of her citizens to the Black Death during the first year of the epidemic. Then followed a remarkable disappearance of the disease from Western Europe. The eighteenth century was marked by few epidemic appearances of plague. At the end of the first half of the nineteenth century it had practically disappeared from Egypt and from European and Asiatic Turkey, formerly its favorite haunts. In interior Asia it has probably existed for centuries, the non-emigrating character of the people limiting and confining its devastations. To these centres and to the commercial invasion of China, we must probably trace the beginning of the present pandemic of plague, which exists to-day, a menace to the civilized and uncivilized world. In the days of the Crusades a religious invasion of the infected centres caused the disease to spread throughout Christendom, while in the present day a commercial invasion has caused it to spread completely around the world. That this is a truth and not a fanciful statement is shown by the appearance of plague in the following countries since 1894, when it spread from interior China. In every case it has followed those sanitary lines of least resistance, the paths of commerce.