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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 infectious
Contemporary Examples
Historical Examples
  • "Evidently, of an infectious fever as your Eminence so well says," he replied.

  • As the doctor declared it not infectious, I was allowed to nurse him.

    Wilfrid Cumbermede George MacDonald
  • The laugh was so infectious that even Brown was obliged to smile.

    The Woman-Haters Joseph C. Lincoln
  • It automatically cleanses the wound of any infectious matter.

    'Smiles' Eliot H. Robinson
  • Her happiness was infectious, and Sarah turned to her visitor with an amused smile.

    Sarah's School Friend

    May Baldwin
British Dictionary definitions for infectious


(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 infectious

"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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infectious 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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infectious 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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