infectious

adjective

in·​fec·​tious in-ˈfek-shəs How to pronounce infectious (audio)
1
a
: producing or capable of producing infection
bacteria and other infectious agents
b
: caused by or resulting from an infection with one or more pathogenic agents
infectious mononucleosis
see also infectious disease
c
: transmitting or capable of transmitting infection : containing pathogenic agents which may be transmitted
infectious droplets
2
: spreading or capable of spreading rapidly to others
an infectious laugh
Her happiness was infectious.
infectiously adverb
infectiousness noun

Did you know?

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 According to the agency, the infectious insects were found in Huntington Beach during routine testing in the region. Jireh Deng, Los Angeles Times, 10 July 2024 Free-range animals are more prone to pick up infectious Toxoplasma oocysts in outdoor pastures and water sources, which have a higher likelihood of being contaminated with cat feces. Bill Sullivan, The Conversation, 10 July 2024 Reba McEntire is remembering Joe Bonsall for his infectious smile. Daniela Avila, Peoplemag, 10 July 2024 The rest of the beauty comes from the ocean’s adorable creatures, which are enthusiastically studied by a group of lovable coral nerds with an infectious passion. Tomris Laffly, EW.com, 10 July 2024 See all Example Sentences for infectious 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'infectious.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Etymology

infecti- (in infection) + -ous

First Known Use

1534, in the meaning defined at sense 1a

Time Traveler
The first known use of infectious was in 1534

Dictionary Entries Near infectious

Cite this Entry

“Infectious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/infectious. Accessed 15 Jul. 2024.

Kids Definition

infectious

adjective
in·​fec·​tious in-ˈfek-shəs How to pronounce infectious (audio)
1
a
: capable of causing infection
infectious viruses
b
: capable of being spread by infection
infectious arthritis
2
: capable of being easily spread
infectious laughter
infectiously adverb

Medical Definition

infectious

adjective
in·​fec·​tious in-ˈfek-shəs How to pronounce infectious (audio)
1
: producing or capable of producing infection
bacteria and other infectious agents
… virus particles, or virions, are protected within these polyhedra and can remain infectious for years outside cells, even in harsh environmental conditions.Felix A. Rey
2
: transmitting or capable of transmitting infection : containing pathogenic agents which may be transmitted
infectious droplets
3
: caused by or resulting from an infection with one or more pathogenic agents
infectious mononucleosis
see infectious disease
infectiously adverb
infectiousness noun

More from Merriam-Webster on infectious

Last Updated: - Updated example sentences
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