=Definition.=—Plague is primarily a disease of rats or other rodents and is caused by a bacterium of the haemorrhagic septicaemia group, _Bacillus pestis_. The disease exists in the rodent in both an acute and chronic form. Acute plague of the rat is apt to be septicaemic, so that when certain species of fleas which infest the rat feed on the blood of their host they ingest plague bacilli. These seem to multiply in the region of the proventriculus and cause thereby an obstruction to the stomach. As a result the flea makes vigorous and repeated but ineffectual efforts to feed. Regurgitation of the contents of the oesophagus occurs, thereby inoculating plague bacilli. When the rats die these fleas will attack man and cause human plague. The ordinary type in man is bubonic plague, characterized by extremely tender glandular enlargements. This form of the disease is thought to be exclusively transmitted from man to man by fleas or possibly bedbugs. A second type is pneumonic plague which is a surely fatal pneumonia which is transmitted from man to man by droplets of sputum expelled in coughing. Either the bubonic or pneumonic types may become septicaemic or this form may exist from the start. Plague shows a marked clouding of the consciousness from the onset and is characterized by toxic action on the heart and endothelial lining of capillaries. =Synonyms.=—Oriental Plague, Black Death, Pestis. French: La Peste. German: Die Peste. HISTORY AND GEOGRAPHICAL DISTRIBUTION =History.=—Ancient writers were accustomed to apply the designation “plague” to any disease which was epidemic in character and attended with great mortality. This explains why the plague of Athens and that of Marcus Aurelius, which epidemics did not possess the characteristics of oriental plague, were so designated. There exist however writings which show that fatal epidemic diseases attended with buboes and prostration were noted prior to the Christian era. It is probable that the biblical description of a disease among the Philistines which was attended with buboes and killed the mice of the field referred to plague. In the 6th century, during the reign of Justinian, a disease which was unmistakably plague started from Egypt and reaching Constantinople caused the death of 10,000 persons in one day. It spread throughout the entire Roman empire. _The “Black Death.”_—The most noted epidemic of plague was that of the “Black Death” of the 14th century. The disease seems to have originated in the East, possibly in China, and eventually invaded Asia Minor, Egypt and Europe. The disease was called “Black Death” in Germany, on account of the petechial spots or “tokens” and in Italy, the “Great Mortality.”