Certain places, at particular times, are infected with maladies which attack a greater or less number of those living or visiting there. Some of these diseases are said to be endemic; that is, they are limited to quite clearly defined places. Thus, ague or malarial fever and autumnal bilious or remittent fever are found to prevail in some neighborhoods every fall and spring; while other places, perhaps not more than a mile distant, are clear of them. Yellow fever is an endemic disease of the vicinity of the sea-coast of Cuba, while the higher regions of the same island are free from it. Cholera is endemic only in Hindustan, near the banks of the Ganges River.

When these, or any other diseases, overpass limited places, and appear in many localities, they are said to be epidemic. Yellow fever is often epidemic. Cholera, once in several years, starts out from India, and travels mostly westward.

Plague was once universally, and is now generally, believed to be extremely contagious.

Erysipelas and puerperal fever cannot be positively said never to be extended from one person to another. Diphtheria, likewise, is sometimes given by one person to another; Usually, however, diphtheria is either a local endemic or a slowly migrating epidemic disorder.

Influenza is always an epidemic; nobody imagines it to be contagious from person to person. The same rule is also of dengue, the " breakbone fever" of the Southern States, and of a form of dysentery prevalent during the summer and autumn in some localities.