=Geographical Distribution.=—At present there are only two important foci of endemicity, one Mesopotamia and another in the region of the Himalayas (India, Thibet and Yunnan). There also seem to be less important centers in Uganda, in Africa, and in the trans-Baikal region of Siberia. In view of the rather widespread infection of the California ground squirrels, from which rodents the disease has from time to time spread to man, it would seem probable that California might be considered another focus of plague. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—The bacillus of plague was first isolated by Yersin from a plague bubo, in 1894, at Hong Kong. It is true that Kitasato reported a bacillus which he had isolated from the blood of a plague patient, on July 7, 1894 (Yersin’s report was made July 30, 1894). Kitasato’s bacillus was motile, Gram-positive, coagulated milk and gave a turbidity in bouillon, characteristics which were just the opposite of those of the organism reported by Yersin. [Illustration: FIG. 63.—Pest bacilli from spleen of rat. (Kolle and Wassermann.)] As now recognized the plague bacillus, when in smears from pathological material, shows the form of an oval bacillus, the ends of which stain more intensely than the central portion (bipolar staining). When cultured on ordinary agar, the morphology is more rod-shaped with a tendency to pleomorphism. These agar cultures are very sticky and mucilaginous. If 3% of NaCl is added to the agar, this pleomorphism is exaggerated, there occurring coccoid, root-shaped and various bizarre involution forms. For obtaining the involution-form appearance on 3% salt agar one should transfer liberally from the ordinary agar growth to the salt agar rather than planting direct from the pathological material. A bouillon culture, upon the surface of which there has been deposited drops of oil or melted butter, shows a string-like downward growth from the under surface of the oil globules. This “stalactite” growth is very fragile and is difficult to obtain. Ordinary bouillon cultures show a rather powdery deposit at the bottom and a hanging-drop preparation from such a culture shows chains of plague organisms resembling streptococci. Gelatine is not liquefied. _Bacillus pestis_ grows readily at room temperature as well as at 37°C, and one may be struck with the fact that colonies on agar plates may show variations in degree of development so that the suspicion of a contaminated culture may arise. Human plague material for cultures or smears is best obtained from the bubo prior to suppuration in bubonic plague, from the blood in septicaemic plague and from the watery sputum in pneumonic plague.