1. communicable by infection, as from one person to another or from one part of the body to another: infectious diseases.
  2. causing or communicating infection.
  3. tending to spread from one to another: infectious laughter.
  4. Law. capable of contaminating with illegality; exposing to seizure or forfeiture.
  5. Obsolete. diseased.

Origin of infectious

First recorded in 1535–45; infect(ion) + -ious
Related formsin·fec·tious·ly, adverbin·fec·tious·ness, nounnon·in·fec·tious, adjectivenon·in·fec·tious·ly, adverbnon·in·fec·tious·ness, nounun·in·fec·tious, adjectiveun·in·fec·tious·ly, adverbun·in·fec·tious·ness, noun
Can be confusedcontagious infectious (see synonym study at contagious)

Synonyms for infectious

Synonym study

1. See contagious. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for infectiousness

Contemporary Examples of infectiousness

Historical Examples of infectiousness

  • And again, as in the case of Tolstoy and his canon of infectiousness, we must ask, What soul?

    Parallel Paths

    Thomas William Rolleston

  • The duration of infectiousness may be longer, although it is not the rule.

  • The second discovery which I shall mention is that of the infectiousness of tuberculosis.

  • Of the infectiousness of nervous disease it seems almost useless at present to speak, for few will listen.

    The Modern Malady

    Cyril Bennett

  • And not only is infection a sure sign of art, but the degree of infectiousness is also the sole measure of excellence in art.

British Dictionary definitions for infectiousness


  1. (of a disease) capable of being transmittedCompare contagious
  2. (of a disease) caused by microorganisms, such as bacteria, viruses, or protozoa
  3. causing or transmitting infection
  4. tending or apt to spread, as from one person to anotherinfectious mirth
  5. international law
    1. tainting or capable of tainting with illegality
    2. rendering liable to seizure or forfeiture
Derived Formsinfectiously, adverbinfectiousness, 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

Word Origin and History for infectiousness



"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

infectiousness in Medicine


  1. Capable of causing infection.
  2. Caused by or capable of being transmitted by infection.
  3. Caused by a pathogenic microorganism or agent.
Related formsin•fectious•ness n.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

infectiousness in Science


  1. Capable of causing infection. See Note at contagious.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.