To determine with certainty whether a disorder which prevails in any place is the plague, it must have all the symptoms which I have just described in one or more patients. These symptoms taken singly, do not constitute the plague; for many other disorders are equally rapid in their course; petechiæ appear in common putrid fevers; in some malignant fevers carbuncles are met with; buboes are produced by the venereal disease and scurvy; and some times, though very rarely, a crisis happens in putrid fevers by abscesses forming under the arm-pits; but these abscesses arise later in these cases than they do in the plague, and moreover they are not accompanied with buboes and the other symptoms which characterize the plague. The high degree of contagion by which the disorder is propagated from one person to another, enters necessarily into the definition of the plague; without it there is no plague. In a word, if there is a frequent communication, either by commerce or in consequence of war, with Turkey or Egypt, and some persons, or a great number of persons, are attacked with a disorder which corresponds exactly to the definition above given, it is certain that it is the plague. 3. For the answer to this third question, the reader has only to revert to the description of symptoms in note A of the Addenda. As for the prognosis, it is attended with great uncertainty in cases of the plague. In some instances, an indisposition apparently slight, is quickly followed by death; whilst others who seem to be on the point of death, recover[62]. In general, when the buboes suppurate well, and there is a separation of the eschars from the carbuncles, accompanied with an abatement of the other symptoms, a favourable prognostic may be given. 4. That hitherto medicine had done very little good, the disorder being so rapid in its course as not to allow time for the remedies to act; but that the Peruvian bark and mineral acids, in large doses, ought, in my opinion, to form the basis of the curative treatment. From the preceding history of the plague it appears, that those who are attacked with this disorder are affected with nervous symptoms before the fever comes on, and that the fever itself is of a highly putrid nature, accompanied with marks peculiar to itself, and which distinguish it from all other fevers. The proportion of those in whom the plague appears under the form of an inflammatory fever, is very small: and this happens only in the beginning of the disorder, in plethoric subjects; and that in these instances, from being inflammatory it quickly becomes putrid. Thus there are two sets of symptoms in the plague, viz. those which depend on nervous irritation, and those which depend on the putrid condition of the blood. The first I call the _nervous_, and the second the _putrid state_.