in·fec·tious | \in-ˈfek-shəs \

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

With inventive flows, razor-sharp wordplay, and an infectious, laid-back delivery, Reyna quickly made a name for herself within Monterrey’s scene. Andrea Alonso, Los Angeles Magazine, "Why You Should be Listening to Mexican-Born, L.A.-Based Rapper Niña Dioz," 5 June 2018 Although hives are not a common flu symptom, Pritish K. Tosh, MD, a Mayo Clinic infectious disease physician and researcher, says it’s possible hives could signal an infection. Char Adams,, "Mom Warns Parents About Rare Flu Symptom After Sick Son Develops Hives: It's 'Very Dangerous'," 1 Feb. 2018 The drug subsidiary focuses on finding treatments for psychiatric and neurological disorders; blood cancers; and kidney, cardiovascular, and infectious diseases., "The week in business," 13 July 2018 The actors still carry scripts, though that is no bar to Mr. Mac’s dangerous enthusiasm or Ms. Blackwell’s mild-mannered insanity or Ms. Celik’s infectious disdain. Alexis Soloski, New York Times, "Review: ‘Gone Missing,’ Now a Poignant Reminder of a Life Cut Short," 12 July 2018 Grandberry is a rising star who bears watching; here her soulful voice, zest for life and infectious humor steal the show. Mike Fischer, Milwaukee Journal Sentinel, "Door County theaters add stories and sparkle to summer nights," 11 July 2018 Perez had to have a leg surgically drained as an infectious disease doctor investigated the cause of the infection. CBS News, "Man could lose limbs after contracting flesh-eating bacteria in New Jersey waters," 11 July 2018 Survivors arrived at the camps with a host of overlapping health issues— tuberculosis, starvation, infectious skin conditions — that were exacerbated by deficient care, said Jael Geis of the Jewish Museum in Berlin at the conference. Emanuella Grinberg, Smithsonian, "When a Bavarian Monastery Provided a Home to Jewish Refugees," 11 July 2018 The onboard audio is punctuated by Jack-Jack’s infectious giggles. Arthur Levine, USA TODAY, "Disneyland's new Pixar Pier: Is it Incredible?," 10 July 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

14 Oct 2018

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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 \

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 \

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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Comments on infectious

What made you want to look up infectious? Please tell us where you read or heard it (including the quote, if possible).


evasion of direct action or statement

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