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.

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.

Extension.—To the eastward, from China, it spread to Japan, the Philippines, Australia, the Hawaiian Islands, Alaska, California, Mexico, Peru and the western coast of South America. To the westward, it invaded India, Mauritius, Egypt, Suez ports, Eastern, Central and South Africa, Mediterranean ports, Great Britain (Scotland), the West Indies and Brazil. In the last twenty years plague has caused millions of deaths, and, during a single week in April, 1907, it destroyed more than 75,000 lives in India, a number about equal to the deaths of a year in London during the Great Plague of 1665. In contrast with India the rest of the world has suffered little during the present world-epidemic, but this loss, while relatively small, is enormous when translated into lives and dollars. The figures for India are simply huge.

Mortality.—The official lists of deaths in India for the last twenty years include some in which the number of reported deaths per year exceeded one million, and it has been estimated that the actual number of persons dead from the plague during this period approximates 8,000,000.

It is gratifying to note a marked decrease in the total mortality in the reports of the last few years, but so long as the annual death list, year after year, was measured by hundreds of thousands, rather than thousands, the situation could not be considered as anything but grave.

Widespread Dissemination in Recent Years.—Without going into statistics deeply we may consider also the list of countries, states and islands from which plague cases have been reported officially during the last few years.

My purpose is to invite attention to the continued existence of various plague foci, any one of which might serve to extend the infection further, were governmental quarantine and public health supervision relaxed.

During August, September, October, November and December, 1909, plague cases occurred in India, Mauritius, China, Japan, Egypt, Turkey, Russia, British East Africa, the Azores, Venezuela, Ecuador, Peru, Chili, California (two cases), and the Hawaiian Islands.

During the first half of 1910 no very marked variation in the distribution of plague occurred, cases being reported from practically all of the foreign countries just named.

A year later the situation, so far as the distribution of plague cases is concerned, was not greatly changed, as may be seen from the following tabulation, which I have abstracted from the British Medical Journal of September 16, 1911.

India.—Deaths from plague in India during the first six months, 604,634. Most prevalent (1) United Provinces, 281,317; (2) Punjab, 171,084; (3) Bengal, 58,515; (4) Bombay Presidency, 28,109. Deaths in July, not included above, 8990.

Hong Kong.—April 24 to August 21, 255 cases, 194 deaths.

China.—January 1, 1911, plague was reported in varying intensity in (provinces and towns) Manchuria, Peking, Tien-tsin, Chefo, Shantung, Shanghai, Amoy, Foochow, Swatow, Canton, Pakhoi and Laichow.

Indo-China.—At Saigon, in March and April, 1911, many cases reported. April 17 to May 7, 56 cases; 17 deaths. May 22 to May 28, 37 cases; 12 deaths.

Siam.—In Bangkok plague was more severe during 1911 than in any previous year. March 15 to April 15, 33 cases and 29 deaths.

Java and Sumatra.—In Java, May 25 to June 3, 105 cases and 62 deaths (one province). In Sumatra plague was present, no statistics.

Straits Settlements.—A few cases, mostly imported, reported in 1911.

Japan.—A few cases at Kobe in 1911. In Formosa, from April 2 to April 15, 31 cases; 24 deaths.

Egypt.—Plague reported from Port Said, Suakin (on board ship), Cairo and Alexandria; also from 11 provinces. The province of Kena had a severe outbreak, May 5 to May 31, 51 cases and 49 deaths.

Persia.—Several cases reported from ports on the Persian Gulf.

Turkey in Asia.—A few cases at Muscat, Basra and at Port of Jeddah.

British East Africa.—Kismayu and Port Florence reported a few cases in April, 1911.

Mauritius.—January 1 to April 11, 110 cases and 70 deaths.Portuguese East Africa.—Plague was reported present at Nahoria in May, 1911.

Russia.—In the Kirgis Steppe in the Astrakan Government in January, 50 cases; 30 deaths.

South America.—Plague prevailed during 1911 in Peru, Ecuador, Brazil, Chile and Venezuela. No severe outbreak except in Peru, where from February to May many cases occurred and died. At Libertad, in March, were reported 60 cases and 23 deaths.

Appearance of Plague in Porto Rico, New Orleans and Manila.—The developments of 1912, which most concern us, were the appearance of human plague and the discovery of plague-infected rats in Porto Rico, Cuba, and the Philippines, and the discovery of infected rats in New Orleans. Thus the Atlantic cities of the United States were for the first time seriously threatened, and the menace of the pestilence at home loomed up on our horizon with sufficient prominence to excite public concern. Our protectors and guardians of the United States Public Health Service, to whose watchfulness we must credit our prolonged escape from the plague, are carrying out all the protective measures at their command with the utmost activity.At the present time we find Porto Rico freed from the disease. New Orleans has undergone and is still undergoing treatment which may be expected, most confidently, to clear it of both human and animal plague.

Of Manila and the work there, much will be found in the following pages, but as both rat plague and human plague have been absent for more than a year we may fairly look upon the epidemic as ended. After so long an interval as this any reappearance of plague may fairly be viewed as a new epidemic, although it is not humanly possible to say that rat plague has entirely and permanently disappeared from the city of Manila, as yet.