Infection never prevails to such a degree, as to affect every person indiscriminately who is exposed to it. Even where the plague and small-pox prevail to the greatest degree, there are some persons who, though susceptible of these diseases, yet escape them. There are certain other infections of a weaker nature, as was before observed, and these will remain entirely inactive, till they find constitutions so disposed as to be fit subjects of their action. The seeds of disease may be compared to those of vegetables, which lye dormant, unless they happen to fall into a situation peculiarly adapted for exciting their activity. It is very difficult to account for this uncertainty in the operation of infection, but it is extremely providential, that under the most calamitous state of sickness, there are always some who are in health and who survive, for the necessary purposes of life. If this were not the case, it might happen that every person on board of a ship might perish from sickness in the course of a voyage, a circumstance which I believe has never been known to happen.
There is an endless variety in the constitution of the human frame, both in mind and body, as well as in the features of the face. There are, perhaps, no two individuals in the world in whom the same effect precisely is produced by the same food, air, medicine, poison, or passions of the mind. The different effects of infection, therefore, upon different people, seem to depend, in many cases, on peculiarities of constitution too obscure to be explained; but there are also known circumstances which resist or encourage its effects.
The great power of habit[61] in taking off the effect of infection, has already been mentioned, and it would appear that novelty gives an increased energy and activity to all impressions, as well as those on the senses. If a person, therefore, escapes the first attack of infection, he will be more likely to continue exposed to it with safety in future.
There are certain precautions necessary to be attended to by those who are unavoidably exposed to contagion, particularly in the first instance. Those who can afford a full diet, and a liberal use of wine, have been observed to resist infection better than those who use food and drink that is meagre and watery. It is also a good rule not to go among the sick, nor otherwise to expose one’s self to infectious air, with an empty stomach; for whether it is that the body is then more susceptible, or that the pores of the skin and lungs are in a more highly absorbing state, so as with greater readiness to inhale the poison of disease, it is certain that a person in that situation is more apt to catch harm from foul air of any kind. Whatever else weakens and exhausts the body, renders it also more susceptible of noxious impressions. Under the head of weakening powers, I comprehend not only what empties the body of its fluids, such as loss of blood, or a diarrhœa, but intoxication, fatigue, fasting, watching, and certain affections of the mind, such as care and grief.
Cold and moisture may also be enumerated among the causes that invite the attack of infectious diseases. They are of themselves simply productive of catarrhs, rheumatisms, and the like disorders; but if an infection should be accidentally present when the body is exposed to them, then instead of these complaints, the disease peculiar to that infection will be produced[62]. This was illustrated in the last reinforcement we had from England; for while bad fevers were breaking out in most of the other ships, the [63]Union was affected with those complaints only which are simply the effects of cold and moisture. It would be more proper, perhaps, to say, exposure to the air, than to call it cold; for exposing the naked body to the open air, even in the warmest climate, is prejudicial to health. This holds at least with regard to Europeans who are accustomed to clothing, however the natives of hot climates who are naked, may expose themselves with impunity.
It is of the greatest consequence to ascertain the extent of the influence of infection, for the means of avoiding and preventing it will very much depend upon this. It is now known, that infection extends itself to a very small distance. There are, indeed, some morbid poisons, such as that of the bite of a mad dog, and that of the venereal disease, which require actual contact to make them take effect. Others are more volatile, and seem to he inhaled by the breath, or absorbed by the skin, but these do not extend far. That of the plague[64] does not reach above a few yards, and that of the small-pox and of fevers is probably equally limited. This discovery is very valuable, by ascertaining the limits of danger; for when a person imagines he runs the same risk when at a considerable distance from the seat of disease, as if he were in contact with the person affected, he will be apt to expose himself wantonly and unnecessarily to the infection.
It seems to be owing to the ignorance of the extent of its influence, that the plague has in general been so fatal; for in consequence of the opinion that the whole surrounding atmosphere was affected, it was vainly attempted to purify it by large fires in the open air, or by [65]firing off artillery, instead of trusting to the separation of the sick so as to avoid their near approach, and to the confinement of those in health to their own houses, which are all the precautions necessary to prevent its progress.