The propagation of disease by contagion, in the modes already stated, though it can take place, and though it sometimes does take place, still there are the strongest grounds for supposing it a comparatively rare occurrence.
I have already shewn, at the beginning of this work, that in one form, the atmospheric, contagion never operates, and I am now prepared to assert, that in the two forms in which alone it can act, that the instances of its undoubted agency are by no means nearly so common as they are commonly believed to be, especially in connexion with those acute diseases, accompanied with fever.
It is my belief, founded on much observation, study, and reflection, that almost all cases of those contagious diseases, arise from causes or circumstances connected with those great agencies already detailed at full length, as inductive of pestilence in general, and of a nature epidemic, endemial, meteorological, and the like.
I am led to the opinion, that this course of origin, even in contagious diseases, is the rule, and that the origin of disease, by contagion, whether contactual or mediate, is the exception. The grounds of this opinion are,—
1st, A fact well ascertained, and of which I had lately two instances, in houses contiguous. Infants neither inoculated, nor vaccinated, lie with their mothers and others ill of small-pox, and do not take that distemper.
2d, Women, while labouring under small-pox, occasionally bear children in perfect health.
The above are common occurrences, and I am in possession of the particulars of several which came under my own observation in the beginning of 1838.
These cases prove the occasional, nay, the frequent inactivity of contagious poison, even when applied in a palpable form, and in a recent condition, to the bodies even of those who are not protected against its operation by inoculation for cow or small-pox, or by a previous attack of disease; and this inactivity is observed too, when the most ample opportunity is afforded for the action of the poison, viz. while children are asleep together in the same bed, and when infants are upon the breast of mothers affected with small-pox.
Those very children who thus escape taking disease by contagion, are frequently known to be seized with that identical disease, at some future time, varying from months to years, when no other case is known to exist in the neighbourhood, and where there is no room to suspect the operation of contagion.
It is, I believe, as common as the contrary course, for small-pox, and other reputed contagious diseases, after appearing in one house in a town or hamlet, to break out in others at a distance and in different directions, and not to progress from that which was first attacked to those lying adjacent, or to spread around as from a centre.