The diseases of animals are in many ways closely linked with those of man. In the case of the larger parasites, such as the tapeworms and the trichina, there is a direct interchange of disease with animals, certain phases of the life cycle of the organisms are passed in man and others in various of the domestic animals. A small inconspicuous tapeworm inhabits the intestine of dogs and seems to produce no ill effects. The eggs are passed from the dog, taken into man, and result in the formation of large cystic tumors which not infrequently cause death. Where the companionship between dog and man is very close, as in Iceland, the cases are numerous. Most of the diseases in animals caused by bacteria and protozoa are not transmitted to man, but there is a conspicuous exception. Plague is now recognized as essentially an animal disease affecting rats and other small rodents, and from these the disease from time to time makes excursions to the human family with dire results. The greatest epidemics of which we have any knowledge are of plague. In the time of Justinian, 542 B.C., a great epidemic of plague extended over what was then regarded as the inhabited earth. This pandemic lasted for fifty years, the disease disappeared and appeared again in many places and caused frightful destruction of life. Cities were depopulated, the land in many places reverted to a wilderness, and the works of man disappeared. The actual mortality cannot be known, but has been estimated at fifty millions. Plague played a large part in the epidemics of the Middle Ages. An epidemic started in 1346 and had as great an extension as the Justinian plague, destroying a fourth of the inhabitants of the places attacked; and during the fifteenth and sixteenth and seventeenth centuries the disease repeatedly raised its head, producing smaller and greater epidemics, the best known of which, from the wonderful description of De Foe, is that of London in 1665, and called the Black Death. Little was heard of the disease in the nineteenth century, although its existence in Asia was known. In 1894 it appeared in Hong Kong, extended to Canton, thence to India, Japan, San Francisco, Mexico, and, in fact, few parts of the tropics or temperate regions of the earth have been free from it. Mortality has varied greatly, being greatest in China and in India; in the last the estimate since 1900 is seven million five hundred thousand deaths. The disease is caused by a small bacillus discovered in 1894 which forms no spores and is easily destroyed by sunlight, but in the dark is capable of living with undiminished virulence for an indefinite time. The disease in man appears in two forms, the most common known as bubonic plague, from the great enlargement of the lymph nodes, those of the groin being most frequently affected. The more fatal form is known as pneumonic plague, and in this the lungs are the seat of the disease.