Note 46 — CHAP. I. AIR.

There are some contagious diseases which cannot be propagated but by their own peculiar infections, as has been before observed, just as the seeds of vegetables are necessary to continue their several species; so that if the infectious poison were lost, so would the disease. Of this kind are the small pox, and the other diseases to which man is subject but once during life. There are other diseases which produce infection without having themselves proceeded from it. Of this kind are fevers and fluxes.

But there is no infection of any kind, however virulent, that affects indiscriminately all persons exposed to it. If a number of persons, who never have had the small pox, are equally exposed to it, some will be seized, while others will escape, who will be affected at another time, when they happen to be more susceptible. It is doubtful how far the habit of being exposed to such specific infections renders the body insensible to them, as was said with regard to fevers; but there is another principle of the animal œconomy laid down and illustrated by Mr. Hunter, which goes at least a certain length in explaining this variable state of the body with respect to its susceptibility of infectious diseases. This principle is, that the body cannot be affected by more than one morbid action at the same time. If a person is exposed to the small pox, for instance, while he labours under a fever, or while he is under the influence of the measles, he will not catch the first till the other has run its course. It may happen, therefore, that people escape the effect of contagion in consequence of being at the time under the influence of some other indisposition, either evident or latent: and supposing the body to be exposed to a number of noxious powers at the same time, one only could take effect. But it seems difficult to explain why some of those who are actually seized, and who have previously been to all appearance in equally good health, shall have it in a very mild degree, while in others it will be malignant and fatal. This is very remarkable with regard to the small pox, which are in some cases so slight, that they can hardly be called a disease, while in others they are so malignant, as hardly to admit of any alleviation from art. May not this, in some measure, be explained from some of the principles above mentioned, in the following manner:—The small pox, in their mildest form, are attended with little or no fever, which, therefore, is not essential to them; and when we see them attended with various forms of fever, and thereby prove fatal even in the most hale constitutions, we ought not to attribute this to any thing in the nature of the small pox, but rather to say, that they have served as an agent in exciting a fever, for which there happened to be some previous latent disposition, that would not otherwise have exerted itself, and that this disposition, or contamination, as it may be called, may have been induced by some past exposure to morbid effluvia, which either from habit, or some other circumstance, may not have been sufficiently powerful to excite the constitution to fever without some such stimulus. Any other occasional circumstance producing disturbance or irregularity in the functions of the body, may, in like manner, excite any particular kind of fever to which the body may at that time be disposed. Thus the amputation of a limb will have this effect; also exposure to cold or fatigue, and intemperance in eating or drinking.

It would appear from these considerations, that there are certain circumstances, or temporary situations of constitution, which invite infection, and render its effect more certain and violent in one case than another. There are artificial methods, however, of obtruding it, as it were, upon the constitution, though not particularly disposed, or even though averse to receive it; and may not this, in some measure, account for the greater safety of some diseases when communicated by inoculation, than when caught in the natural way?

But these, as well as many other facts in animal nature, do not admit of a satisfactory explanation upon any principle as yet known. Even the most common operations of the body, such as digestion and generation, when considered in their causes and modes of action, are so obscure and mysterious, as to be almost beyond the reach of rational conjecture. A little reflection will teach us the utmost modesty with regard to our knowledge of such things; for nature seems to have innumerable ways of working, particularly in the animal functions, to which neither our senses can extend, nor perhaps could our intellects comprehend them. Had we not, for instance, been endowed with the sense of sight, nothing could have led us even to suspect the existence of such a body as light; and there may be numberless other subtile and active principles pervading the universe, relative to which we have no senses, and from the knowledge of whose nature and exigence we must for ever be debarred. We have, indeed, become acquainted with electricity by an operation of reason; and animals have lately been discovered to which the electric fluid serves as a medium of sense through organs calculated to excite it, and to receive and convey its impressions.

But there are few subjects we can study that are more subtle and obscure than the influence of one living body on another. There is a familiar instance of the great subtilety of animal effluvia, and also of the fineness of sense in a dog’s being able to trace his master through crowds, and at a great distance; and we can conceive that infectious matter may adhere, and be communicated in a similar manner. We have endeavoured to illustrate the great obscurity of its operation by an allusion to generation, digestion, and other animal functions, with which it is equally obscure and inexplicable. It is similar to generation in this, that its influence does not pass from one species of animal to another; for the poison of the plague, that of the small pox, that of fever, and the venereal disease, do not affect brutes47, nor do the infectious diseases of brutes affect different species of them, nor the human species. The only exception to this, that we know of, is the bite of a mad dog.

From these facts, and also from what was formerly mentioned of contagion not affecting indiscrimately all that may be exposed to it, it would appear that some nice coincidence of circumstances is necessary to modify an animal body, so as to receive its action. There must be a sort of unison, as it were, or sympathy, betwixt different living bodies, so as to render them susceptible of each other’s influence.

It is none of the least curious facts with regard to infection, that there are some species of it by which the body is liable to be affected only once in life. When this is considered, it is indeed conformable to what happens in the course of the disease itself; for, unless there was in the body a power of resisting it, there could be no such thing as recovery. Where the disease actually exists, the continued presence of the poison, which is also infinitely multiplied, would infallibly prove fatal in all cases, unless the living powers were to become insensible to it48.