Ships of war are exposed to infection chiefly by receiving such men as have been raised by pressing, who are frequently confined in guardships, under such circumstances of bad air and bodily filth as tend to generate the most virulent infection. The service also requires sometimes that men be received from jails, and they are either criminals delivered over by the civil jurisdiction of the country, or captives who have been restored by the enemy after a course of confinement in their prisons. It may happen too, as we have seen[43], that the enemy, who are made prisoners at sea, may have infection about them, and will communicate it the more readily that they are strangers. There are few fevers but what are infectious at some stage or other of the disease; but it is not necessary that fever should actually exist in order to create infection. In the most violent and pestilential fevers, such as have sometimes originated in the jails of England, the persons who communicated them were not affected with it themselves[44]. Infection, like some other poisons, does not affect those who are accustomed to it, and therefore those who are in the habit of being exposed to it frequently escape its bad effects, especially if it is gradually applied, as must be the case with those about whose persons it is generated. For the like reason, physicians and nurses are less susceptible than others; and strangers, who are accustomed to a pure air, are the most susceptible of any. It is observed by Dr. Short, that contagious epidemics are more frequent and fatal in the country than in London, and this may probably be accounted for on the same principle; for every person in a great town is exposed to the breath and effluvia of others, and to a variety of putrid exhalations, which are unavoidable where multitudes inhabit together; but they are so used to them, that they are not affected by them; whereas in the country, where people are less accustomed to each other’s company, and less used to impure air in general, they are the more readily affected when infection is introduced among them. It may even admit of a doubt if any society of men, living together, are entirely free from morbid contagion. It certainly sometimes happens, that a ship, with a long-established crew, shall be very healthy; yet, if strangers are introduced among them, who are also healthy, sickness will be mutually produced. This principle in the human constitution, by which the presence of strangers affects it, is well illustrated by a fact[45], founded on the best testimony, that, in one of the small western islands of Scotland, which is so remote, that the inhabitants are frequently without any communication with strangers for several months together; they become so susceptible, in consequence of this long interruption of intercourse, that they are seized with a catarrh when strangers of any description come among them. It was said before,