Epidemic Diseases (Gr. upon, and people) are those which attack at the same time a great number of persons in a given locality, depending on some temporary, accidental, and generally inappreciable cause ; differing in this respect from endemic diseases, or those developed under the influence of some constant or periodic cause. Many diseases, ordinarily sporadic, may become epidemic under certain ill-understood conditions ; or some new disease, introduced by contagion or other favorable circumstances, may spread epidemically. Having ascertained the cause, or the epidemic tendency of the season, the treatment must depend on the nature of the disease and the constitution of the patient; even when remedial measures seem powerless, the physician can do much to check an epidemic by inspiring confidence and moral courage, and by withdrawing the attention of a community from the continual consideration of any supposed causes. The human constitution may become acclimated to epidemic diseases in malarious climates, as is shown by the greater mortality among new comers.
In the white races there is no acclimation against endemics of intermittent and bilious fevers and other marsh diseases, as the experience of our southern states and the Pontine marshes of Italy fully proves. Negroes to a certain extent become insusceptible to the effluvia of the rice fields, but not so much so to the causes of disease on the cotton plantation; they suffer more than whites from cholera, typhoid diseases, plague, and smallpox, but are much less liable to intermittents, and the smallest admixture of negro blood is a great protection against yellow fever. (See Acclimation.)