_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. For example, the first case of Typhus Fever which occurred in my practice, in January 1839, was at Meadow Mill, a village half a mile north of Tranent; the second case was at a hamlet called Redcoll, about four miles east; the fourth and fifth cases occurred in Tranent; while the sixth and last for that month appeared at Elphinstone, a village situated about two miles to the south-west of Tranent. I am led also to the opinion, that the ordinary cases, even of those diseases which are known to be occasionally propagated by contagious poison, do not arise from contagion, but from other circumstances and agencies; by the history of the plague, for while that scourge is ravaging in the East, and destroying hundreds daily, it frequently ceases, immediately upon the overflowing of the Nile, which buries and covers the pestiferous soil, and the putrefying materials which had been exhaling noxious emanations.