To give a full and satisfactory answer to the questions here proposed would involve so much professional and physiological detail, as would be unsuited to the character of such a publication as "N. & Q." I will therefore content myself with short categorical replies, agreeable to the present state of our knowledge of these mysteries of the animal economy. It is true as a general rule that the infectious diseases, particularly the exanthemata, or those attended by eruption--the measles for example--occur but once. But there are exceptional cases, and the most virulent of these non-recurrent diseases, such even as small-pox, are sometimes taken a second time, and are then sometimes, though by no means always, fatal. Why all the mammalia (for, be it observed, these diseases are not confined to the human race) are subject to these accidents, or why the animal economy should be subject to such a turmoil at all, or, being so subject, why the susceptibility to the recurrence of the morbid action should exist, or be revived in some and not in others; and why in the majority of persons it should be extinguished at once and for ever, remain amongst the arcana of Nature, to which, as yet, the physiology of all the Hunters, and the animal chemistry of all the Liebigs, give no solution. Those persons who take note of the able, and in general highly instructive, reports of the Registrar of Public Health, will observe that the word _zymotic_ is now frequently used to signify the introduction into the body of some morbific poisons,--such as prevail in the atmosphere, or are thrown off by diseased bodies, or generated in the unwholesome congregation of a crowded population, which are supposed to act like yeast in a beer vat, exciting ferments in the constitution, in the case of the infections diseases, similar to those which gave them birth. But this explains nothing, and only shifts the difficulty and changes the terms, and is no better than a modification of the opinions of our forefathers, who attributed all such disorders to a fermentation of the supposed "humours" of the body. The essence of these changes in the animal economy, like other phenomena of the living principle, remain, and perhaps ever will remain, an unfathomable mystery. It is our business to investigate, as much as in our power, and by a slow and cautious induction, the laws by which they are governed. Non-recurrence, or immunity from any future seizure in a person who has had an infectious disease, seems derivable from some invisible and unknown _impression_[4] made on the constitution. There is good reason to suppose that this impression may _vary in degree_ in different individuals, and in the same individual at different times; and thence some practical inferences are to be drawn which have not yet been well advanced into popular view, but to which I cannot advert unless some reader of "N. & Q." put the question. M. (2)