(from contagio—con together, and tango to touch). This term is used to express the propagation of diseases of a specific nature from one individual to another, and also the matter by which this propagation is effected. In the latter sense contagion is a morbid agent, sui generis, which on being introduced into the blood, produces a definite train of morbid phenomena, and communicates to the blood the property of generating a similar poison, capable of originating a similar disease if introduced into the blood of another individual. Diseases so propagated are termed contagious, and the matter by which they are propagated is called contagious matter, or simply contagion.

Strictly speaking, the term contagious ought to be limited to those diseases which are alone propagated directly by actual contact of the body which receives the poison with that which produces the poison. It has, however, become customary to class under the same term diseases which are also propagated by means of the poison of the contagion being diffused through the air. Contagious diseases, therefore, are divisible into two great classes,—1st, those which are alone capable of being propagated from person to person by the direct application of the contagious matter—immediate or contactual contagion; and 2d, those which are communicable in this way, and also through the medium of the air—mediate or remote contagion. To the first class belong itch, ringworm, syphilis, cow-pox, &c. To the second belong smallpox, scarlet fever, measles, typhus fever, &c.

The term infection is frequently and improperly confounded with, and used synonymously for, the term contagion. But this term is not properly applicable to diseases, but to the effects produced by malaria or effluvia arising from marshes, or that from decaying animal and vegetable substances.