In the London Bills from 1681 to 1684, the deaths from fever were many, with some from “spotted fever” nearly every week, while the annual mortalities from all causes were high. It is the more remarkable, therefore, that Sydenham should have discovered, in the beginning of 1685, the outbreak of a new fever, different from any that had prevailed for seven years before. The explanation seems to be that a malignant typhus fever, such as might have been discovered in any year in the crowded parishes where the working classes lived, broke out at the Court end of the town, where Sydenham’s practice lay. The epidemic fever of 1685-86. A letter of 12 March, 1685, says: “Sir R. Mason died this morning in his lodging at Whitehall. A fever rages that proves very mortal, and gives great apprehensions of a plague[32].” Sydenham also was reminded of the circumstances preceding the Great Plague of London in 1665. In his first account of the epidemic of fever in 1685[33], which began with a thaw in February, he points out that the thaw in March, 1665, had been followed by pestilential fever and thereafter by the plague proper. In a later reference, when the epidemic of fever was in its second year (1686) he says: “How long it may last I shall not guess; nor do I quite know whether it may not be a certain more spirituous, subtle beginning, and as if _primordium_, of the former depuratory fever (1661-64) which was followed by the most terrible plague. There are some phenomena which so far incline me to that belief[34].” However, no plague followed the malignant, if not pestilential, fever of 1685-86. The reign of plague, as the event showed, was over; the fever which had been on former occasions its portent and satellite, came into the place of reigning disease. It is true that Sydenham does not identify the fever of 1685-86 by name as pestilential fever; on the contrary, he entitles his essay “De Novae Febris Ingressu.” But the novelty of type was partly in contrast to the fevers immediately preceding, which admitted treatment by bark, and its principal difference from the pestilential fever of former occasions seems to have been that it was not followed by plague[35]. Its antecedents and circumstances were very much those of plague itself. Its mortality was greatest in the old plague-seasons of summer and autumn, it had slight relation to famine or scarcity, or to other obvious cause of domestic typhus. Sydenham can find no explanation of the new constitution but “some secret and recondite change in the bowels of the earth pervading the whole atmosphere, or some influence of the celestial bodies.” He enlarges, however, on the character of the seasons preceding, which would have affected the surface, if not the bowels, of the earth, and the levels of the ground-water.