in·​fec·​tious | \ in-ˈfek-shəs How to pronounce infectious (audio) \

Definition of infectious

1a : capable of causing infection viruses and other infectious agents
b : communicable by infection : caused by pathogenic microorganisms an infectious disease — compare contagious
2 : that corrupts or contaminates
3 : spreading or capable of spreading rapidly to others their enthusiasm was infectious an infectious grin

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Other Words from infectious

infectiously adverb
infectiousness noun

What is the Difference Between contagious and infectious?

The words contagious and infectious can be confusing because they do not designate wholly distinct categories; something is not either contagious or infectious.

Essentially, infectious diseases and contagious diseases are caused by disease-producing agents such as bacteria and viruses, but they differ in that contagious diseases can be spread to other people by direct or indirect contact.

Anything contagious, such as the flu, is always automatically infectious: if you can catch it from someone, it's being passed to you via an infectious agent, which is the thing that gets you sick—usually a virus or a bacteria.

The reverse, however, isn't true. Just because something is infectious does not mean it's contagious. Food poisoning, for example, is infectious but not contagious: food can be contaminated with a bacteria (an infectious agent) that makes you sick, but you can't give your food poisoning to someone else by shaking their hand or even giving them a kiss.

Both contagious and infectious are also used figuratively, often in much happier contexts: laughter can be contagious; someone's enthusiasm can be infectious. While both words are used figuratively of both pleasant and unpleasant things, contagious is more often chosen for the unpleasant, as when it's grumpiness or fear that seems to be spreading.

Examples of infectious in a Sentence

viruses and other infectious agents She has an infectious grin.

Recent Examples on the Web

And its music and sultry black-and-white cinematography are infectious. Alissa Wilkinson, Vox, "“I wanted to make it a beautiful disaster”: Pawel Pawlikowski on his new film Cold War," 21 Dec. 2018 With added size and power, Siquieros is looking to anchor the defensive line as the Colts make the difficult jump from 3A to 5A. His passion and work ethic are infectious. Richard Obert, azcentral, "Top 10 Arizona high school football defensive tackles for 2018," 12 July 2018 His smile and laugh were infectious and he will be forever missed. Hartford Courant,, "Jason Robert Horan," 1 July 2018 Their stamina was apparently infectious, bolstering Simon’s energy time and time again. Zack Ruskin, San Francisco Chronicle, "Paul Simon bids a fond ‘Farewell’ at Oracle Arena with final Bay Area concert," 26 May 2018 The series kicks off on June 21 with a performance from indie rocker Cary Brothers and a sure-to-be infectious sing-along screening of The Greatest Showman; check back for updates on the rest of the lineup. Marielle Wakim, Los Angeles Magazine, "Outdoor Movies Collide with Concerts at This New Rooftop Summer Series," 10 May 2018 The trend may yet be infectious; a poll conducted by Vanderbilt University found that nearly 40% of Brazilians, exhausted by crime and corruption, would support a military coup in their country. Ian Bremmer, Time, "The 'Strongmen Era' Is Here. Here’s What It Means for You," 3 May 2018 The tune is infectious—an enviable blend of retro flair and hip-hop flavor that feels very now. Christopher Rosa, Glamour, "These Will Be the Only Songs You'll Hear This Summer," 9 Apr. 2018 Mussolini’s pleasure in language is nevertheless infectious. Colin Dickey, The New Republic, "Why Dictators Write," 22 Mar. 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'infectious.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of infectious

1534, in the meaning defined at sense 1a

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Last Updated

18 Mar 2019

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The first known use of infectious was in 1534

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More Definitions for infectious



English Language Learners Definition of infectious

: capable of causing infection
: capable of being passed to someone else by germs that enter the body
: suffering from a disease that can be spread to other people by germs


in·​fec·​tious | \ in-ˈfek-shəs How to pronounce infectious (audio) \

Kids Definition of infectious

1 : passing from one to another in the form of a germ an infectious illness
2 : easily spread to others an infectious laugh


in·​fec·​tious | \ in-ˈfek-shəs How to pronounce infectious (audio) \

Medical Definition of infectious

1 : capable of causing infection a carrier remains infectious without exhibiting signs of disease viruses and other infectious agents
2 : communicable by invasion of the body of a susceptible organism all contagious diseases are also infectious, but it does not follow that all infectious diseases are contagious— W. A. Hagan — compare contagious sense 1

Other Words from infectious

infectiously adverb
infectiousness noun

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