Definition 1: Communicable; able to carry or transmit disease.

Usage 1: In a strict sense, and in keeping with its etymology, "contagious" is used to describe diseases that are transmitted by close contact, in contrast to "infectious" diseases, transmitted by air or water: SARS is infectious; AIDS is contagious. In common usage, however, "infectious" and "contagious" are considered synonymous. Something contagious is a "contagion"; it exhibits "contagiousness" and behaves "contagiously."

Suggested usage: When used figuratively, it seems that "infectious" is generally applied to good things like laughter, good humor and enthusiasm. "Contagious" can cover these, too, but it more often deals with negative emotions: "In the Stock Market today, the panic was contagious." Other types of contagion abound: "I'm afraid the new recruit has just been exposed to the contagion of Steve's disillusionment."

Etymology: From the Latin prefix con- "with," implying closeness, and "tangere," to touch; "contingency" and "contiguous" have the same derivation. A "tangent" is a line that touches a curve at only one point, and something "tangible" is touchable. "Tactus," the past participle of "tangere," gives us "tactile," relating to touch, "tact," a delicate emotional touch, and "intact," untouched. Finally, there is "tax," a touch that none of us seems to be able to avoid.