“Contagion,” as its etymology implies, means the communication of disease from person to person by contact. It is often confused with “infection;” but it has quite a different meaning. Contagion presupposes the existence of certain germs, like the sporules of fungi, which can be bottled up and conveyed any distance, attached to clothing, merchandize, especially to woollen stuffs, for which it is supposed to have a particular affection, and to feathers, which of all articles it especially loves, so much so that, according to Quarantine laws, a live goose may be safely introduced from a plague country; but, if it happens to be eaten on the voyage, its feathers cannot be introduced without danger to the entire community. The absurdities connected with the doctrine are numberless. Suffice it to say that in the ordinary sense of the term there is no such thing as “contagion.” There are two or three diseases in which there is a specific virus which can be seen, tasted, smelt, and analysed, and which, in certain constitutions, propagates the original disease by inoculation, such as small-pox, cow-pox, and syphilis, but these are not “contagions” in the sense supposed. With regard to the mysterious, imponderable, indivisible nonentities, which make up our “current contagions,” they may safely be dismissed into the limbo of extinct superstitions. The word “contagion” therefore is altogether objectionable. The word “infection” expresses a fact, without involving a hypothesis. It is most necessary, however, that the meaning should be guarded; for, just as there is no such thing as “contagion,” there is no such thing as _inevitable_ “infection.” Infection acts through the air. Poison the air breathed by individuals and we have infection. Shut up 150 healthy people in a Black Hole of Calcutta, and in twenty-four hours you have an infection produced, so powerful that it will in that time have destroyed nearly the whole of the inmates. Sick people are more susceptible than healthy people; and if you shut up sick people together, without a sufficient space and sufficient fresh air, you will have not only fever, but erysipelas, pyæmia, and the usual tribe of Hospital-generated epidemic diseases produced. Again, if we have a Fever Hospital with over-crowded badly-ventilated wards, or with the air stagnant in them, we are quite certain to have the air become so infected as to poison the blood not only of the sick, so as to augment their mortality, but also of the medical attendants and nurses, so that they shall also become subjects of fever.