The histories of different ages and countries furnish numerous records of the occasional prevalence of certain diseases, generally of the febrile class, which at one period have occasioned the most destructive mortality, while at others, they have assumed so mild a form as to have affected only few, and to have destroyed scarcely any of the population. Such diseases, when they attack a great number of individuals about the same time, or in rapid succession, are very properly designated by the term EPIDEMIC (from επὶ _upon_ and δημος _the people_) and whenever their course is attended with considerable mortality, they are moreover said to be _Pestilential_. No fact in the history of medicine has been the subject of more general and anxious enquiry, or of more keen controversy, than that of the origin of Epidemic diseases, and of the immediate cause of their propagation and decline; and although the field has been industriously explored by the most able and experienced philosophers and physicians, the subject still remains involved in considerable obscurity; indeed, such different and even opposite views have been entertained upon the question, that writers have not even agreed upon the exact import of the terms employed in their descriptions, but each author appears to have acknowledged a latitude of acceptation according to the particular theory which he has been anxious to support. The term EPIDEMIC ought in strictness to signify a disease which, as we have before stated, attacks numbers at or nearly the same time, without any reference to the cause from which it may have originated, or be diffused; but this construction has been considerably limited by many writers, who have applied it, exclusively, to denote those maladies which derive their origin solely from a noxious state of the atmosphere, and which are incapable of being communicated from one person to another; distinguishing diseases of the latter kind by the epithet _Contagious_.[170]