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.