In addition to the wide-spread desolating epidemics which appear from time to time, mysterious in their origin, progress, and cessation or disappearance—such, for example, as the plague of Athens, the plague of London in the time of Charles the Second of happy memory, the Indian or Asiatic cholera of modern times, and the disease called influenza, a frequent visitor to Western Europe during the last half-century—there exist localities unceasingly under the influence of a poison inimical to human life. This poison, since it may be so called, is known to haunt the deltas of large rivers, and seems to be always present there; but it is found also, if we may determine its identity by the identity of its deleterious influence on men, in other and very various localities: sometimes it shows itself—and this most commonly—in marshy and fenny countries, where no large rivers exist, at other times by the banks of fresh-water lakes; now it haunts the forest, and now the open plain, where marsh and fen, swamp and decaying vegetation, seem all but absent. As the inhabitants of such localities are especially afflicted with the fevers called intermittent and remittent, it is the most natural thing in the world to ascribe to the locality itself the origin of these diseases. When, however, we attempt to generalize and assign to the same cause in a more concentrated form those terrible fevers which render tropical countries the graves of Europeans, great difficulties arise, and numerous objections, which the best of statisticians, not to mention the simply medical observer, have failed to elucidate and remove. Thus physicians are not agreed as to the identity of the poison under all circumstances, or in other words, demonstrative evidence is still wanting to prove that the cause of fever on the western coasts of Africa is identical with that which has so often in the Antilles destroyed England’s chosen troops, decimated her fleets, crippled her power, annihilated her army, as at Walcheren, and broken up the health of many a sturdy yeoman by the banks of the Scheldt, of the Thames and its tributaries. To this poison the term malaria has been applied—a word borrowed from the Italian. This malaria is presumed, whatever it may be, to be the cause (though not exclusively), on evidence almost amounting to a certainty, of the fevers marked by intermissions and remissions; it may also be the cause of the more terrible febrile diseases called the yellow fever, the black vomit, &c., of tropical countries. On this I do not insist. As regards intermitting and remitting febrile affections, we are all but certain that to such localities as I have just alluded to, their origin may be traced, however they may originate elsewhere. A long residence in Holland and Belgium (countries supposed by many to be in an especial manner the hot-bed and active parent of malaria) has enabled me to observe, I trust in an unprejudiced manner, some facts which may have escaped the observation of others. Long resident in that land, on which perished miserably the best equipped army (an army composed of veterans) which ever, perhaps, quitted England for foreign aggression; in that land on which perished the chosen garrisons of the mighty Napoleon; on that spot where they dragged on a miserable existence, or perished in the prime of life; the writer of this essay enjoyed the best of health. Even admitting the full influence of a vigorous constitution, and an innate vitality equal to the neutralization of all malaria, a something must still be ascribed to observation leading him to avoid the hurtful and insalubrious agencies at work around him—agencies ever active, ever seeking to destroy. This information the author has thought might be useful to others, and with this view he submits it to the public.