Epidemic

Epidemic (from two Greek words epi, upon, and demos, a people), a term applied to a disease which attacks whole communities and is capable of spreading so eis to involve, it may be, the inhabitants of countries widely separated from one another; for example, the Asiatic plague, the Black Death of the 14th century, cholera, and epidemic influenza are all instances of epidemic diseases. It was at one time the fashion to ascribe epidemics to atmospheric influence, and it was customary to use the term "epidemic constitution" to indicate the peculiar conditions of climate and of the atmosphere, which were supposed to determine the invasion of communities by particular epidemic diseases. The doctrine of the germ theory of disease has considerably modified these older views. The reason why the spread of certain diseases is so much greater at some times than at others remains, however, in great part unexplained.