Etiology. — Small animals, monkeys, squirrels, rats, etc., die in great numbers during epidemics of plague, rats par ticularly—and they seem to be first to suffer. This fact has suggested that the specific organism discovered by Kitasato, or that described by Yersin, might be of telluric origin. Whether this be the case or not, there is much evidence in favor of the contention that plague is a soil bred and soil-supported disease. The bacillus of plague has been found in the soil and in the dust of houses inhabited by sufferers. This micro-organism once transported to a suitable soil may there flourish and form foci of infection. Ac cording to Robert Koch (Dent. med.. Woch , July 14, 'OS), there are three en demic main plague foci in Asia: Meso potamia, Thibet, and Assir, while he locates the primary source of the disease in the English territory of Uganda.
There does not appear to be a neces sary connection between the infection of animals and outbreaks among men. Al though small animals and insects, includ ing the domestic fly and the flea, die of the disease and may spread it, it seems more logical that general insanitary con ditions, enhanced by the long prevalence of drought, should tend to awaken the latent activity of the germ.
After the activity of the plague ba cillus has reached a certain potency, through appropriate surroundings, hu man beings are assailed as well as the lower animals, the latter succumbing first, owing to their greater exposure to toxic germs through promiscuous feed ing. The tissues most vulnerable to the Kitasato bacillus are the skin and the mucous membrane, especially when these are deprived of their protective covering. Yamagiwa (Annales de l'Inst. Pasteur. Aug., '97) found a wound of the surface in one out of every seven cases examined. The next portal of invasion is believed by some to be the alimentary and respi ratory tracts. Once beneath the surface, the bacillus is thought to penetrate the lymphatic system and thence to invade the system at large.
The development of the bubo is one of the earliest symptoms; it precedes the profound disturbance of the nervous system. The disease originates in au "infection" through the skin, remaining localized for a time. The systemic or general infection develops from a local centre or focus of origin. Report of Austrian Plague-Commission (Philadel phia Med. Jour., Jan. 28, '99).
The plague bacillus is not devitalized by a three weeks' exposure to a tempera ture of 24° C. below zero. This bacillus
can develop slowly, but steadily, at a temperature of from 20° C to 27' C. Near the freezing-point it remains inert. Chinese houses, with their bad ventila tion and other unhygienic conditions, fur nish favorable conditions for the develop ment of the plague bacillus. N. Noguchi (Sei-i-Kwai Med. Jour., May 31, 1900).
Although all classes suffer, certain con ditions of life appear to confer immunity. Persons living in-doors are more likely to suffer than those who are often exposed to the sun's rays. The boating popula tion of China who live exclusively upon the water seldom suffer. Persons who occupy the upper stories of a dwelling are less frequently attacked than those living upon the ground-floor. The infec tion may be transmitted by means of body-linen, clothes, bedding, rags, bag ging, carpets, etc. Foodstuffs, grain, sweetmeats, etc., are also thought capa ble, under suitable conditions, of trans mitting infectious germs. Few nurses or attendants upon the sick are attacked if their habits are cleanly; even those whose duty includes the disinfection of infected dwellings have been free from the disease when personal cleanliness ob tained. During the epidemic in Canton, during which upward of 30,000 Chinese died, not one of the 300 American or English residents, according to the China Medical Missionary, was affected.
Pathology. — In the buboes and the glands involved Childe (Brit. Med. Jour., Sept. 24, '9S) found bacilli in enormous numbers both among the cells of the gland-tissue and among the lymphatic vessels and the blood-corpuscles extrava sated into the gland, as well as in the hemorrhage outside the gland. In cases of plague-septicemia they were similarly present in the large characteristic glands. In the kidney the bacilli could also be seen, especially among the blood-cells of the tubules into which hemorrhage had occurred. In the spleen they were also present, among the cells of the splenic tissue and in the hemorrhagic areas. They were present in the liver, especially in cases in which engorgement and hem orrhage were marked. The bacilli were present in the pneumonic areas of plague pneumonia, in profusion among the ca tarrhal epithelial cells and leucocytes that fill the alveoli and terminal bron chioles, as well as among the blood-cor puscles of the alveoli into which limmor Hinges occurred.