14 TEE POPULAR SCIENCE MONTHLY 



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 ante- 

 cedents 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, kill- 

 ing 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 



