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[in-fek-shuh s] /ɪnˈfɛk ʃəs/
communicable by infection, as from one person to another or from one part of the body to another:
infectious diseases.
causing or communicating infection.
tending to spread from one to another:
infectious laughter.
Law. capable of contaminating with illegality; exposing to seizure or forfeiture.
Obsolete. diseased.
Origin of infectious
First recorded in 1535-45; infect(ion) + -ious
Related forms
infectiously, adverb
infectiousness, noun
noninfectious, adjective
noninfectiously, adverb
noninfectiousness, noun
uninfectious, adjective
uninfectiously, adverb
uninfectiousness, noun
Can be confused
contagious, infectious (see synonym study at contagious)
1. catching.
Synonym Study
1. See contagious. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for infectiously
Contemporary Examples
Historical Examples
  • Zizi laughed outright, so infectiously, that Douglas joined in.

    The Come Back Carolyn Wells
  • Is there anything in life so infectiously joyous as a wagging tail?

    Balloons Elizabeth Bibesco
  • They assembled themselves together of set purpose to be lively, and they were infectiously dismal.

  • Here in Christmas time you will witness many jolly and infectiously pleasant scenes.

  • Under a soft panama hat a keen, shrewd face smiled so infectiously that the disgruntled seaman smiled back in spite of his grouch.

    Gold Out of Celebes Aylward Edward Dingle
  • We laughed at that: men awaiting death on the battlefield laugh easily, though not infectiously.

  • Here the Reverend John Middleton, unmistakably and infectiously happy, talked with his supposed niece for an hour.

    Elsie Marley, Honey

    Joslyn Gray
  • He laughed as only a negro can laugh, and he kept it going so infectiously that Tom and I got started, just watching him.

    Pieces of Eight Richard le Gallienne
  • He never said anything deliberately funny except on the stage, but laughter emanated from him infectiously, as yawning might.

British Dictionary definitions for infectiously


(of a disease) capable of being transmitted Compare contagious
(of a disease) caused by microorganisms, such as bacteria, viruses, or protozoa
causing or transmitting infection
tending or apt to spread, as from one person to another: infectious mirth
(international law)
  1. tainting or capable of tainting with illegality
  2. rendering liable to seizure or forfeiture
Derived Forms
infectiously, adverb
infectiousness, noun
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for infectiously



"catching, having the quality of spreading from person to person," 1540s of diseases, 1610s of emotions, actions, etc.; see infect + -ous.

Online Etymology Dictionary, © 2010 Douglas Harper
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infectiously in Medicine

infectious in·fec·tious (ĭn-fěk'shəs)

  1. Capable of causing infection.

  2. Caused by or capable of being transmitted by infection.

  3. Caused by a pathogenic microorganism or agent.

in·fec'tious·ness n.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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infectiously in Science
Capable of causing infection. See Note at contagious.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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