In ordinary cases the bacillus is found in buboes. A _bubo_ is nothing more than an inflamed gland. In so-called septicæmic cases it is found in the blood of the animal afflicted by the disease. In pneumonic cases the bacillus may be found in the sputum of the patient. It is the custom to speak of (_a_) bubonic plague, (_b_) septicæmic plague, (_c_) pneumonic plague, as though they were three diseases. This is inaccurate: for they are only forms, with varying symptoms, of one and the same disease caused by the same bacillus. The disease which we call plague is, in truth, really a fight between the afflicted animal and the invading bacillus. It may be inferred from the fact that bacilli are scarcely ever found in the blood in bubonic cases that the invaders are stopped by the lymphatic glands next above the point of inoculation. In such cases the fight, which is the illness, takes place chiefly in the bubo. In non-bubonic cases the fight goes on in the blood-vessels or in the lungs as the case may be. Whether the plague is primarily a disease of rats would be difficult to say; but rats and other rodents are very susceptible to it. It has also been transferred to mice, rabbits, guinea-pigs, squirrels, pigs, sheep, goats, cattle and horses. Men and monkeys are equally susceptible. Cats and dogs have been known to die of it and during the Great Plague of London many were destroyed under the belief that they were bearers of infection. That plague among human beings was associated with mortality among rats and mice, is an observation of great antiquity. The student of the Hebrew scriptures will remember the Book of I Samuel vi. 4: “Then said they, What shall be the trespass offering which we shall return to him? They answered, Five golden emerods [buboes] and five golden mice [rats] according to the number of the lords of the Philistines: for one plague was on you all and on your lords.” Eastern authors, of a later date, refer in several places to rats, in times of plague, staggering about as though they were drunk. The Mogul Emperor Jehangir in his diary of the plague at Agra (1618) mentions the unusual mortality of the rats. In India it seems long to have been a custom, dictated by experience and caution, to leave houses when rats began to die. In Europe, during the middle ages, the mortality of rats when the plague was raging does not seem to have impressed the chroniclers and during the recent outbreak at Glasgow (1900) none was detected. As an illness of mankind, the plague reached Europe from the East. We have no evidence of any outbreak in Europe before the reign of the Emperor Justinian. When it raged for the first time at Constantinople (A.D. 542) the mortality was enormous. Ten thousand persons are said to have died in a day with all the symptoms of bubonic plague.