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 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. 2019

Examples from the Web for infectiously

Contemporary Examples of infectiously

  • He could be infectiously enthusiastic and extraordinarily charming: whenever we talked, his eye contact bordered on hypnotic.

    The Daily Beast logo
    Is Steve Jobs Obsolete?

    Alan Deutschman

    June 8, 2009

Historical Examples of infectiously

  • 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?


    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

British Dictionary definitions for infectiously



(of a disease) capable of being transmittedCompare 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 anotherinfectious mirth
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 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

Medicine definitions for infectiously




Capable of causing infection.
Caused by or capable of being transmitted by infection.
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.

Science definitions for infectiously



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.