Endemic (from en, 'among,' and dēmos, 'the people'), a term applied to diseases which affect numbers of persons simultaneously, in such manner as to show a distinct connection with certain localities. Endemic diseases are usually spoken of as contrasted with Epidemic (q.v.) and Sporadic (q.v.); endemic indicating that a disease infests habitually the population within certain geographical limits, and also that it is incapable of being transferred or communicated beyond those limits; while, on the other hand, a disease is termed epidemic if it is transmitted without reference to locality, and sporadic if it occurs in isolated instances only. It should be mentioned, however, that it is quite possible for an endemic disease afterwards to become epidemic, and many of the best known forms of complaint, such as cholera, yellow fever, &c., have a well-defined local habitat or place of incubation, in which they originate and subsequently spread to surrounding districts. The theory, accordingly, of endemic diseases is, that they are in some way or other connected with the soil—the result of terrestrial influences, or miasms—of poisons generated within the earth, or near its surface, and diffused through the air, so as to be weakened in proportion to the distance from the source of the poison. Such poisons are always observed to be more virulent in summer than in winter—more dangerous at night, when the vapours are concentrated on the surface of the soil, than in the daytime—more abundant in the plains, and in close confined places, than at a certain degree of elevation—more easily carried in the direction of the wind than in the opposite—and very often arrested altogether by water, or by a belt of forest or other luxuriant vegetation. In all these particulars, endemic are different from epidemic diseases, which bear no very obvious relation to the soil, and are not observed to be considerably modified either by the prevailing winds or the period of the day or night at which exposure to their influence takes place.
The most marked type of an endemic disease is Ague (q.v.) or Intermittent Fever, which has all the habits mentioned above, and is to so marked a degree a denizen of particular tracts of country as to lead to their being in some instances almost depopulated. Many places in Italy are a prey to the aria cattiva or Malaria (q.v.), as it is popularly called; and hence, no doubt, even more than for protection from human foes, the custom so prevalent in that country of building the villages on the tops of hills, so as to secure immunity from the poisonous vapours raised by the solar heat from the plains lying on either side at the base of the Apennines. Terrestrial miasms, or such poisons as generate endemic diseases, are usually found in the neighbourhood of marshy flats, or of uncultivated tracts of land at the confluence of rivers, or where a delta, or a wide channel subject to overflow, is formed at the upper end of a lake. In proportion, too, as the heat of the sun is greater, the tendency to malarious emanations is increased; and in the tropics, accordingly, large tracts of jungle and forest are often rendered absolutely uninhabitable and almost impassable at certain seasons by the invisible and odourless germs of intermittent, remittent, and even continued Fevers (q.v.), which are more fatal and unmanageable than the most terrible epidemic pestilences to those who are exposed to them. Such diseases are almost always sudden in their mode of attack, and they indicate the range of their influence by the number of persons attacked; but they are wholly free in most cases from the suspicion of communication by Contagion (q.v.), which is so frequent in the case of epidemic diseases. The precise nature of the malarious poison, though generally believed to be of the germ order, has never yet been discovered with any approach to exactness. It is known, however, to be almost invariably checked by drainage and cultivation of the soil; and hence many places in Europe, formerly very productive of endemic diseases such as ague, have now ceased to be so, as in the case of the Tuscan Maremma, and some parts of Kent and Essex, and of the Lothians in Scotland. The Eucalyptus-tree (Eucalyptus globulus) is said by several authorities to have a very beneficial influence on malarial districts, but this is a disputed point, and at best is probably quite as much due to the rapid growth and widespread nature of the roots of the Eucalyptus, which thus efficiently drain the soil, as to any effect produced by the essential oils given off in a vaporous condition from the branches.