Contagion

Contagion, the communication of a disease from the sick to the healthy, either by direct contact of a part affected with the disease, or through the medium of the excretions and exhalations of the body. The term is sometimes used of all methods by which disease is communicated, but more generally of those in which direct contact has taken place between the healthy body and the diseased one, or some object which has touched it; more subtle means of transference, especially through the air, being termed 'infection.' This distinction, though it cannot be considered a fundamental one, is practically convenient; for some diseases (e.g. syphilis, ringworm) are always, or all but always, propagated by contact; others (e.g. measles, influenza) much more often through the air; while in some (e.g. anthrax) the symptoms and course of the disease are much modified by the method of communication. See INFECTION, GERM THEORY OF DISEASE.