young sanitary engineers I have known—a student of mine at the Massachusetts Institute of Technology, who with many others was infected in a boarding house by a waitress who was nursing another servant ill with typhoid, in the intervals of her regular domestic duties. It is often impossible to prevent such catastrophes, but the danger should be kept in mind and all possible effort made to ascertain the health antecedents of those whom we take into our households.
The second of the common modes of disease transmission to which I have referred, spread by the agency of insects, is on the whole easier to control and in highly civilized communities is much less important than spread by articles of food. We must bring foods into our homes, and it is often hard to discriminate between the infected and the non-infected. Insects however can be entirely excluded from the household in cities, and may be kept under reasonable control even in the country. The most spectacular triumphs of modern sanitation have been achieved in the war against insect-borne disease and even before their sanitary importance was at all comprehended, rising standards of personal cleanliness, by the elimination of vermin, incidentally caused a marked reduction in diseases of this class. Our medieval ancestors with their rush strewn dining halls and their uncleansed bedding and clothing paid a heavy toll to the insect carriers of disease. Bubonic plague, the terrible Black Death of the Middle Ages, we know to be primarily a disease of the rat, commonly transmitted from rat to man and from man to man by the flea. Two great pandemics of this disease are recorded in history, one beginning in the sixth and the other in the eleventh century. In each case the pandemic started in Asia, spread to Constantinople and then through Europe, to almost all parts of the known world. At the height of the second great pandemic, in the fourteenth century, twenty-five million people—about one fourth of the population of Europe—were swept away and in the London plague year, immortalized by Defoe, 100,000 persons perished. A third great epidemic began in Hongkong in 1894 and again spread in India, killing 6,000,000 people between 1896 and 1907. Since that time infection has spread as far as Australia and Brazil. The rats in certain districts of England and the ground squirrels in California are known to be still infected with the plague bacillus. Yet no epidemics have occurred outside of Asia, simply because the rats and fleas which spread the disease are under control. If we lived in filth, as our forefathers did, there can be no doubt that we should be in the midst of a great world scourge of plague like that of the fourteenth century. So with typhus, or ship, or jail fever, which was one of the serious diseases of Europe and America a hundred years ago. It has now almost disappeared in western Europe and the United States, and its decrease was a mystery until it was shown that the germ is carried by the body louse. Personal cleanliness has automatically wiped out this disease, while typhoid
