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.