[3]. It may be remarked that for some time prior to the Cattle Plague in the autumn of 1865, the disease called pleuro-pneumonia had extensively prevailed among the herds throughout the country. [Ed.]
The second circumstance, and a most instructive one it is, premonitory of the advent of a great Epidemic, is a general transformation of the type of ordinary diseases into the characteristic type of the approaching pestilence. Sydenham gives a graphic description of such a transformation in the character of the fevers and inflammatory diseases prevailing in London some months before the outbreak of the Great Plague. He states that this change consisted in an approximation, in several striking features, of the general type of disease, to the distinguishing characters of the Pestilence which had not yet appeared, but was close at hand.
In 1831, in the wards of the London Fever Hospital, I observed and recorded a precisely similar change in the general type of the fevers in London, six months before the first visitation of Cholera. Anterior to that period, fever in London, for a long series of years, had been essentially an acute, inflammatory disease, for which bloodletting and other depleting remedies were indispensable. At this period it ceased to be an inflammatory disease; it became a disease of debility, in which no one could think of bleeding; and so closely did the prevailing fever now put on the general character of the approaching plague, which was as yet six months distant, that the fever into which those Cholera patients fell, who were not killed by the first stroke—the consecutive fever, as it was afterwards called—could not be distinguished from the primary fever in the wards of the Hospital when Cholera was at its height, which had appeared there for the first time six months previously, and which has never disappeared since.[[4]]
[4]. This was written in November, 1855.
It is further very remarkable that the Professors of Veterinary Medicine and Surgery in London noted at the same time a similar change in the type of the diseases of the lower animals—horses, cows, sheep, and all domestic creatures;—a change requiring a similar modification of the remedies which they had been in the habit of using.
5. A further character of great Epidemics, partly arising from the last, is this:—they are actually present and in operation some time before they assume their distinct and proper form. Sometimes, indeed, the very first cases are most intense and characteristic, but at others they are scarcely to be distinguished from the severer attacks of ordinary disease of a like nature. Hence doubt is sometimes reasonably entertained of their true character. When at length increasing numbers leave no doubt of the actual presence of the dreaded malady, the first announcement of it is always received with incredulity and sometimes with resentment; and so it is that Epidemics always take a country by surprise—burst suddenly on an unprepared people, who wilfully shut their eyes against the plainest evidence, as if they would avert the event by denying its existence.
6. Again, Epidemics resemble each other in the uniformity of their course. They present, with great regularity, periods of comparative quiescence and activity—periods of well-marked increase, culmination, and decrease.
7. They further resemble each other in the manner of their migration. They advance by leaps. On breaking out in a locality they soon come to their height, decline, and disappear. Then they attack another locality; here they pass through precisely the same process as before, and proceed to a third, fourth, or fifth district, and so on. Sometimes indeed they localize themselves on the same spot for a considerable period, and then several places may be simultaneously affected; but for the most part a large city may be regarded as a cluster of towns, through the several districts of which epidemics advance as if they were proceeding from one town or village to another. Hence the duration of an epidemic in a place is generally proportionate to its size. The several localities attacked being visited in succession, a space of time is required to spread through the whole of them proportionate to the magnitude of the town.
8. Epidemics resemble each other in the periodicity of their return.
On its first visitation (1485) the Sweating Sickness spread over the whole of England in the course of one year, when it disappeared.