A disease so fatal in its nature and against which human power is so futile was in ancient times naturally attributed to wrath of the gods. Supernatural, astrological, and, in some instances, rationalistic causes were assigned to it. In the fourteenth century the College of Physicians of Paris ascribed it to the influence of constellations in India. But more natural explanations, however, gradually followed. Putrefaction of dead animals was assigned as a cause in Egypt. Poisoning of water-supply was also believed to be the cause. Undue heat, rain, watery grain, and absence of the Etesian winds were thought to generate plague. Leaving ancient theories on the causation of the disease we find that modern Scientists divide themselves into two classes: _first_, those who believe in the germ theory and attribute the plague to a specific germ, holding that germs can never arise _de novo_; _second_, those who believe that atmospheric changes and certain telluric conditions or insanitary surroundings engender the seeds of pestilence which are carried through air, water or other media. The arguments in favour of the first theory are, however, so strong and overwhelming, that it is now almost universally accepted that plague is due to a specific poison which grows and multiplies under favourable conditions, and that wherever it occurs it is caused by the implantation of those germs in a suitable soil. If the soil is not fit, the germs may be sown, but they will not germinate and, if the soil is fit but the germs are absent, the disease will not be seen. The soil best suited for the plague seed is one where insanitary conditions prevail. Dirt and filth, bad ventilation, and overcrowding are its manure. The history of plague from ancient times fully illustrates that plague thrives in dirt, filth, squalor and misery. Diseased grain and want of subsoil drainage are held to be potent factors in the diffusion of plague. _BACILLUS_—