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

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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 Randy, with his infectious grin, twinkling eyes, warm handshake, and rote recitation of Japanese garden symbology, was not condemned for a string of killings but rather for leading a death squad one summer night in 1987. Matt Thompson, SPIN, 12 Mar. 2024 Four cases have been reported in California, including one in Los Angeles County involving a person who arrived Jan. 25 on a Turkish Airlines flight while infectious and later visited a Chick-fil-A restaurant in Northridge. Rong-Gong Lin Ii, Los Angeles Times, 12 Mar. 2024 Then the pathogens can survive and remain infectious on fomites for varying lengths of time, from a few hours in some cases to several months in others depending on variables related to the pathogen, the fomite and their environmental conditions. Sabrina Sholts, Smithsonian Magazine, 12 Mar. 2024 Though the peak of this winter’s surge is receding, wastewater analysis data suggests there are still more than 760,000 new cases of COVID a day in the US—meaning roughly 1 in 63 people are actively infectious. Reina Sultan, SELF, 7 Mar. 2024 The 5-foot-7 forward with an infectious smile and positive attitude scored the lone goal with 3 minutes, 50 seconds left in Westview’s 1-0 win over Mission Hills in the Southern California Regional Division 2 championship at Mission Hills High on Saturday evening. Jim Lindgren, San Diego Union-Tribune, 3 Mar. 2024 But containment of the highly infectious illness requires special measures, the CDC noted. Helen Branswell, STAT, 1 Mar. 2024 Reaction of Risk Management: Significant roles are played by numerous public sector institutions in the prevention, diagnosis, and treatment of infectious outbreaks. Chuck Brooks, Forbes, 29 Feb. 2024 Pace takes clear, infectious pleasure in such ephemeral moments, combing through bootlegs of live performances like an archivist looking for clues in a medieval manuscript. Owen Myers, Pitchfork, 29 Feb. 2024

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 19 Mar. 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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