Individual predisposition to contract the disease seems to be increased by all depressing influences, among which may be mentioned excessive bodily or mental exertion, intense and prolonged anxiety, fear, and the like. Previous debilitating disease also increases the liability to the attack. Neither sex nor age exerts an influence in this respect, save that after the age of fifty few contract the disease. Occupation confers no immunity. Physicians, nurses, and others occupied in the care of the sick, and those who bury the dead, have especially suffered in recent6 as well as in the older outbreaks. Oil-carriers and dealers in oils and fats, and to a less degree water-carriers and the attendants at baths, are said to enjoy a comparative immunity from attack. Those who have suffered from the disease and recovered also enjoy a relative immunity. Second attacks are usually of less intensity than the first.
6 See summary of a report addressed by Dr. G. Cabiadis to the Constantinople Board of Health on the outbreak in Astrakhan in Russia, 1878-79, by E. D. Dickson, M.D., Medical Times and Gazette, 1881, vol. i. pp. 4, 32, 119.
2. The Exciting Cause.—The exciting cause of the plague must, in the present state of our knowledge, be assumed to be a specific infecting principle. Upon no other hypothesis can the continued existence of a disease so specific in its characters, unchanged through the course of centuries, disappearing when the influences favorable to its presence cease, reappearing in certain regions when they again arise, be explained. Capable of being transmitted by the vehicles of commercial intercourse, of control by quarantine and cordons sanitaires, of spreading from limited foci of contagion into overwhelming epidemics, the plague is the very type of the infective diseases. The nature of this infecting principle is wholly unknown. It is probably a microphyte capable of development within the human organism—capable also of a prolonged independent existence under favorable circumstances outside of the body, and of again giving rise to the disease. The plague is properly to be classed as a contagious-miasmatic disease (Liebermeister) with cholera, dysentery, and enteric fever. It continues to exist by the continuous propagation of its cause, and it spreads by the transportation of that cause.
It is conceded on all hands that the plague has never arisen autochthonously in Europe, but has in every instance been conveyed thither. Those who regard its reappearance after long intervals of time in those countries where it still occasionally prevails as spontaneous are compelled to ignore difficulties in reasoning far greater than the supposition of an equally prolonged condition of quiescence or an inexplicable or unsuspected reintroduction of the cause.
As to the disputed question of the contagiousness of the plague, to set forth the arguments and examples adduced in favor of either view would far exceed the limits of the present article. All the facts are to be explained upon the theory that the exciting cause of the plague, like that of cholera and enteric fever, consists of a miasm that must undergo certain changes outside the body before acquiring its virulent properties, and that the time required for these changes is exceedingly brief. But what the physical properties of this miasm are, or how it finds access to the body, or how it is eliminated, are alike utterly unknown to us.
It is certain, however, that it is incapable of being freely transmitted to great distances in the air. Whether or not it is conveyed or retained by the discharges from the bowel is not known. The history of recently observed outbreaks, from which alone definite and trustworthy facts are to be obtained, goes to show that the exciting cause of the plague clings closely to the patients and their immediate belongings. The closer the relation between those sick and the healthy, the greater the risk that the latter will contract the disease. Those in the house with the patients are more liable to fall sick than those in the adjoining houses—those who are constantly in their presence than those who occasionally see them. Thus, nurses much more frequently contract the plague than doctors, though the latter have in all epidemics been largely numbered among the victims. Among 357 deaths in the outbreak in Vetlanka, already referred to, were a priest, his wife and mother, three doctors, six assistant medical officers, and two Sisters of Mercy. Dr. Cabiadis remarks that the information obtained "shows that the malady propagated itself, in the first instance, from the sick to their relatives and to those who lived with them or who assisted them during their illness. If, on the one hand, these facts showed its contagious character, on the other hand evidence is still wanting to prove whether this transmission of the malady was caused by contact with the sick and their clothing, or by breathing an atmosphere impregnated with the deleterious particles emanating from their morbid bodies."
The period of incubation is from two to seven days. In the report of the commission of the French Academy of Medicine, drawn up by Prus in 1844, the statement appears that the plague has never shown itself among compromised persons after an isolation of eight days. The recent outbreaks tend to confirm this conclusion. L. Arnaud concluded from observations made at Benghazi in 1874 that the mean duration of this period was five or six days, and that the maximum did not exceed eight days. Cabiadis sets this stage down as three days as the rule, but as occasionally not exceeding twenty-four hours. He found no data, however, to show the longest period to which it could extend. Hirsch, from information collected in his investigation of the same epidemic (that of Astrakhan), concluded that the minimum period of incubation observed was from two to three days, the maximum more than eight, and that the average was five days. He states that very short or very long periods were seldom observed.
SYMPTOMATOLOGY.—Individual cases of the plague, as of other epidemic diseases, differ in their onset and progress under different circumstances and at different periods of particular outbreaks. Besides the ordinary form, to which as a type the greater number of the cases more or less closely conform, there are, on the one hand, others so severe that death takes place before the characteristic manifestations have time to appear, and, on the other hand, cases so light that such manifestations are but partly developed, and the nature of the malady is only to be recognized in the light of the prevalent epidemic influence.
Hence among the cases three forms are recognized: (a) The grave or ordinary form; (b) the fulminant form; and (c) the larval or abortive form.
(a) Grave or Ordinary Form.—The plague in typical cases is a febrile malady of the most acute kind, with localizations in the form of buboes or carbuncles.