These, as signified by name, owe their origin to, or are caused by, a peculiar principle to which the name of malaria or miasm has been given. Of the chemical nature of miasm we literally know nothing; but we have abundant evidence that it is a specific cause of disease. There are, practically, two kinds of malaria: First, koino-miasmata, the product of vegetable decomposition, or terrestrial emanations; second, Idio-miasmata, the deleterous effluvia originating from the decomposition of matter derived from the human body. Both of these are prolific causes of disease, yet the profession, owing to the subtile nature of the miasms, are in a great degree ignorant as to the manner of operation. These two causes may act separately, and produce their different symptoms, or they may operate together, causing a confusion of morbid phenomena.

"Marsh gas," or the product of vegetable decomposition, owing to its diversification, is of course the greater cause of disease. Two requisites, heat and moisture, are necessary for its production; and hence, where these abound in any quantity, so proportionably is the miasm evolved. For this reason, low, marshy lands are at certain seasons very unhealthy, while those regions at a greater elevation are peculiarly healthy in this respect. Wherever vegetation is profuse, and to which abundant heat and moisture are contributed, there we may reasonably expect a plentiful product of miasm and consequent disease. Experiments have proved that in decomposition of vegetable matter, animal matter--infusoria--is produced in very rapid succession, having an exceedingly short-lived existence. These infusoria are inhaled at every breath, as the air contains swarms of them, but which are imperceptible to any of the senses. It is reasonable to suppose that they, in a great measure, contribute largely to periodic fevers. The diseases generally classed as Malarial are Intermittent, Remittent, Yellow, and Typhus Fevers.