“I consider it therefore as an established fact in the Levant, that commerce and intercourse with infected towns is sometimes attended with no bad consequence. The same thing may perhaps be asserted, without restriction, of all countries; but till the signs indicating a pestilential constitution be ascertained, no particular year can be declared exempt from danger. Predictions founded on _planetary conjunctions_ have been long exploded; and signs derived from the known properties and alterations of the air, are almost equally fallacious. The seasons concomitant with plagues in England, as well as elsewhere, have been very dissimilar; and the same visible concurrence, usually deemed pestilential, has often, in the revolution of years, been observed to return, in various countries, without producing the dreaded consequences. Upon the whole, from all I have been able to collect, the pestilential constitution seems hitherto to be known only from its effects; _neither its approach nor its retreat can be predicted and its nature remains wrapped up in_ MYSTERIOUS DARKNESS.” Having thus seen, that, of the causes commonly assigned for epidemical diseases, not one can be accounted certain and determinate, it now remains to consider one more, and that is _Contagion._ Though this has been generally accounted the same with _infection_, yet by some it has been reckoned otherwise; and indeed there seems to be a necessity for such a distinction; for, though we should prove, ever so clearly, that a disease once communicated to one person should from that person be communicated to another, yet the difficulty is to know from whence the first person had it. This source, if any such can be found, is what we may with the greatest propriety distinguish by the name of _contagion_, and is the sense in which it shall for the future be used in this treatise, the matter communicated from one person to another being always called _infection_. This indeed differs from what many celebrated physicians have said upon the subject; but the distinction certainly must exist. Dr. Cullen speaks rather indistinctly upon the subject. “We have supposed that _miasmata_ are the cause of intermittents, and _contagions_ the cause of continued fevers, strictly so named; but we cannot with propriety employ these general terms. The notion of _contagion_ properly implies a matter arising from the body under disease, _miasma_, a matter arising from other substances. But, as the cause of continued fevers may arise from other substances than the human body, and may in such cases be called a _miasma_, and, as other miasmata also may produce contagious disorders, it will be proper to distinguish the cause of fevers by using the terms _marsh_, or _human_ effluvia, rather than the general ones _miasma_, or _contagion_.”