Experience has taught us that the phenomena of epidemic pestilences or diseases are various and dissimilar, observing no regular course or succession, but commencing and ceasing at periods influenced by certain changes of the seasons, and modified by various circumstances, especially such as locality and habit of body.
The chronicles of all nations are replete with notices respecting the remarkable commotions of nature, which have proved from time to time so inimical both to the animal and the vegetable kingdoms; and on carefully reviewing the facts detailed in the histories of epidemic pestilences or diseases, it will be readily perceived that, during certain periods or seasons, ELEMENTAL DISTURBANCE has enveloped, as it were, the entire globe, carrying death and misery into every quarter. I am, therefore, of opinion that the grand phenomena of nature, exhibited in the commotions of our physical world, supply us with abundant materials for the explication of all epidemic pestilences or diseases; that the latter are consequently assignable to natural causes, without searching for or hunting after mysterious agencies, to the neglect of those which Nature is constantly presenting to our view.
I am fully aware that ascertaining the causes of maladies of any description is a matter attended with much difficulty, not only from the variety and impalpable nature of the causes themselves, but also from the peculiar functions and various idiosyncrasies of the living system on which such causes act; numerous conditions being found capable of modifying the effects of a common cause of disease, viz. the season of the year, peculiar state of the atmosphere, especially as regards heat and humidity, circumstances of locality, the different temperaments of individuals exposed to their agency, the influence of previous ailments,—all these concurring more or less in the production of diversified forms of disease.
By way of illustration and with reference to atmospheric influence—the grand exciting cause of disease,—every practitioner is familiar with certain conditions which are favourable to the production of certain disorders, such as common catarrh or cold, and its more aggravated form termed influenza,—hooping-cough, bowel affections, gastric fevers, &c. The effects of external temperature first produce on the functions of the skin (which connects us with the exterior world) an augmentation or diminution of the natural discharges, by suddenly changing or disturbing the balance of the circulating fluids between the external and internal parts or surfaces, irrespective of the perspiratory process, commonly termed sweat, there being innumerable chemical agents more poisonous and of greater importance as to their elimination from the system in the shape of insensible perspiration. I repeat, that the effects of external temperature, by causing a repercussion of the cutaneous exhalations (sensible and insensible), so damage the vitality of the system, as to be the exciting cause of disease; and, although from our imperfect knowledge, it scarcely admits of investigation, setting aside chemical research, so that we cannot explain or define the precise peculiarity of each and every condition of the atmosphere favourable to the production of each variety of disorder, the effects of such external temperature are, however, not the less certain and obvious to our senses. The agencies are all natural, comprised in the excess of state or change acting on individuals whose susceptibility varies greatly from moral as well as from physical predisposing causes; but our inability to define their peculiar conditions or auræ, as noticed by Hippocrates, Sydenham, Bacon, and others, no more militates against our reasoning upon their visible effects, than a knowledge of the gravitating principle or power is essential to the proof of gravity.
But to the subject of the causes,—the exciting and the predisposing of disease; it should be premised that when attributing epidemic pestilences or diseases to natural causes, it must not be supposed or inferred that any one of such causes, as heat, cold, moisture, drought, &c., will alone be sufficient to induce disease: to effect that, there must be a combination of these causes in continuance and succession, in occasion and variation, as also in circumstance and virulence.
It has always appeared to me that a proper and sufficient distinction is never, or rather, has never, been made, between the predisposing and the exciting causes of disease—an error which leads to much confusion and to contradictory conclusions on all sides. Par exemple, in a Report on the epidemic pestilence which raged some years ago in Ireland, it was unanimously determined, (so says the Report,) “that the predisposing causes were, rapid atmospherical vicissitude, low marsh and other effluvia.” Here we have the exciting cause, ‘atmospheric vicissitude,’ jumbled together with a predisposing cause, ‘marsh effluvia;’ while on the other hand, in recent Reports on cholera from various authorities, we have marsh and other effluvia assigned as the exciting causes, which, on reflection, will strike any one as being obviously fallacious, inasmuch as if marsh and other effluvia are to be considered as exciting causes, instead of predisposing to disease, we should scarcely ever find those places free from pestilence where these matters are supposed to be engendered, whereas we see various localities in South America,—in British Guiana, for instance, and even in Ethiopia,—which have been condemned as the hot-bed and nursery of pestilence, where putrefaction is supposed to concoct and concentrate its most lethal poisons,—still enjoy their seasons of salubrity: it is, I repeat, from the absence of such distinctions as to the exciting and the predisposing causes, coupled with the non-consideration of the great variety of circumstances, both past and present, local and general, individual and common, which are influential in the production of disease, that so much uncertainty and disputation as to the causes of epidemic pestilences have arisen. Such distinctions will appear to be more important and necessary for the prevention and mitigation of epidemic pestilence or disease, when properly considered, inasmuch as the predisposing causes, which are in a great degree under our control, are always in existence or operation to a greater or less extent, while the exciting causes may be said to be of only occasional occurrence.
As before observed, the causes of diseases are described as the predisposing and the exciting. The former, the predisposing causes, I understand to be,—the want of light; impure air (especially from defective ventilation), in which are included malaria and all other noxious vapours, from whatever source arising; a scanty and impoverishing diet; and, though last, not least, habit of body, induced by the irregular and artificial life of man in a state of civilization, which, by enervating the system, induces the predisposition to disease by rendering the human frame more susceptible to external impressions; the grand excitants of disease, such as elemental disturbance, consisting of variations of temperature and the state of the surrounding medium, as to hygrometric influence, electrical tension, &c.
Naturalists in all ages have concurred in the opinion that sudden atmospheric mutations act injuriously on the subjects of the vegetable kingdom as well as on those of the animal, and it will, I presume, be admitted that the existence of the vegetable kingdom is necessary for the support of the animal,—the physiology of both reflecting much light on each other. We see the vegetable kingdom assailed by disease termed blight,—‘pestiferous blight,’ as it has been called by some; these blights are but pestilences affecting the vegetable kingdom, and have ever been attributed to natural causes, elemental disturbance, &c.
At the periods which have been remarkable for the occurrence of blights, animals—such as horned cattle—have been known to suffer sooner or later; and man also, as I shall in the course of these pages fully show. Reasoning analogically, therefore, may we not conclude that that state or condition of the elements which induces blights,—pestilences in the vegetable,—will also act injuriously on the animal kingdom, more especially on the human race, men being, from habit of body, &c., more prone to or susceptible of disease?
Referring to the annals of history, which so abundantly show that the ancients considered all universal distempers as originating in the disturbed states of the seasons, I will cite a few instances (commencing with sacred history) explanatory of the causes of universal maladies, termed epidemic pestilences or diseases.