Q: A mentor of mine, an MD, taught me that a contagious disease spreads by contact between people while an infectious one doesn’t need contact. This would suggest that laughter or fun is “contagious,” not “infectious.” Other usage gurus are a bit vague, but I hope to find your usual precision on this one.
A: Medline Plus, the online medical dictionary from the National Institutes of Health, generally agrees with your mentor. It defines “contagious” as “communicable by contact,” and “infectious” as “capable of causing infection.”
Of course the two terms aren’t mutually exclusive. As William Arthur Hagan explains in The Infectious Diseases of Domestic Animals (1943), “All contagious diseases are also infectious, but it does not follow that all infectious diseases are contagious.”
Although many people use the two terms interchangeably, the definitions for “contagious” and “infectious” in standard dictionaries are similar to those in medical references.
The American Heritage Dictionary of the English Language (5th ed.), for example, defines “contagious” as “transmissible by direct or indirect contact,” and “infectious” as “capable of causing infection.”
However, both terms have strayed from those senses when used figuratively, according to the Oxford English Dictionary and Fowler’s Modern English Usage (rev. 3rd ed.).
The lexicographer R. W. Burchfield, writing in Fowler’s, discusses the figurative evolution of “contagious” and “infectious”:
“In figurative uses, contagious tends to be used of both pleasant and unpleasant things (in the OED and OED files of corruption, folly, guilt, panic, and suffering, but also laughter, shyness, and vigour), whereas infectious is mainly restricted to pleasant things (enthusiasm, good humour, laughter, sense of fun, simple delight, virtue, and zeal).”
Merriam-Webster’s Dictionary of English Usage agrees that in recent figurative usage “contagious can be used of pleasant and unpleasant things, but infectious is almost always used of pleasant things.”
So, “laughter” and “fun” can be described as either “contagious” or “infectious,” though they’re more likely to be called “infectious.”
Now, let’s look at the history of these two adjectives.
English adapted the word “contagious” from the Old French contagieus in the 1300s, but the ultimate source is the Latin noun contagion, which refers to touching and contact as well as contagion.
In the OED’s earliest example, from Chaucer’s 1374 translation of Boethius’s De Consolatione Philosophiæ, “contagious” is used to describe the corruption of the soul by being in contact with the body:
“Whan I lost my memorie by the contagious coniunccioun of the body with the soule.” (We’ve replaced the runic letter thorn with “th” throughout.)
By the early 1400s, the adjective was being used in its medical sense. in Lanfranc’s Science of Cirurgie (circa 1400), leprosy is referred to as “oon of the syknessis that ben contagious.” (We’ve replaced a thorn here with “th.”)
In the 1600s, according to OED citations, people began using “contagious” figuratively to describe all sorts of things “communicated from one to another or to others.”
The dictionary’s first example is from A Treatise of Seraphic Love (1660), by Robert Boyle: “If our Friends do not allay our Love or Affection by unwelcome Actions, or their contagious Sufferings.”
The next citation is from Milton’s Paradise Lost (1667): “Well understood Of Eve, whose Eye darted contagious Fire.”
As for “infectious,” English adapted the term from Anglo-Norman and Middle French in the 1500s, but the ultimate source is inficere, Latin for to dye, stain, infect, imbue, or corrupt.
Originally, according to the OED, the English term referred to “causing or spreading disease, esp. of an epidemic nature.” Later, it came to be used more widely to mean capable of “causing or transmitting infection.”
The dictionary’s earliest citation for “infectious” is from a 1534 treatise on epidemic disease by Thomas Paynell, an English friar who wrote on literary and medical subjects:
“For from suche infected bodies commethe infectious and venemous fumes and vapours, the whiche do infecte and corrupte the aire.”
Soon, according to OED citations, the term was being used figuratively to mean “tending or liable to infect or contaminate character, morals,” but that sense is now considered rare.
By the early 1600s, the adjective took on its modern figurative sense of “having the quality of spreading from one to another; easily communicable.”
The earliest Oxford example is from The Maid’s Tragedy, a 1619 play by Francis Beaumont and John Fletcher: “She carries with her an infectious griefe, / That strikes all her beholders.”
The next citation, from John Dryden’s Fables Ancient and Modern (1700), uses the term positively: “Through the bright Quire th’ infectious Vertue ran. All dropp’d their Tears.”
We’ll end with a more recent example from a 1988 issue of Rugby News: “He’s very infectious and the sort of guy people want to follow.”
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