infectious

adjective

in·fec·tious | \ in-ˈfek-shəs

Definition of infectious

1a : producing or capable of producing infection bacteria and other infectious agents

b : transmitting or capable of transmitting infection : containing pathogenic agents which may be transmitted infectious droplets

c : caused by or resulting from an infection with one or more pathogenic agents infectious mononucleosis Influenza, malaria, and tetanus are infectious diseases.

2 : spreading or capable of spreading rapidly to others an infectious laugh Her happiness was infectious.

Other Words from infectious Synonyms What is the Difference Between contagious and infectious? More Example Sentences Learn More about infectious

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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

The queen who don't need no crown makes her triumphant Atlantic Records debut after years of dropping infectious one-offs bops. — Cady Drell, Marie Claire, "Sound On: The Best Music of April 2019," 1 May 2019 China, which produced and consumed around half of the world’s pork last year, could face a shortage of the meat because of the disease, which is highly infectious but harmless to humans. ... — Jacky Wong, WSJ, "Swine Fever in China, Stock Fever at WH Group," 10 Apr. 2019 These little infectious particles can remain active in the air for a long time — up to two hours, according to the CDC. — Caroline Picard, Good Housekeeping, "The Measles Signs in Kids Every Parent Needs to Know About," 3 Apr. 2019 This weekend, Kelly Clarkson gave the infectious hit a go—and the Internet promptly lost it over her killer performance. — Christian Allaire, Vogue, "Kelly Clarkson Covered “Shallow” and the Internet Lost It," 17 Feb. 2019 Still, the rare rat case gnaws at the lingering questions of hepatitis E that Meng and other infectious disease trackers have chipped away at for several decades. — Beth Mole, Ars Technica, "Rat hepatitis jumps to human for first time, spotlighting disease mystery," 2 Oct. 2018 The anti-vaccine movement, in which parents refuse to vaccinate their children based on uncredited medical research that claims the injections are toxic and/or can cause autism, has, in part, led to a return of preventable infectious diseases. — Michelle Ruiz, Vogue, "Facebook Just Announced a Plan to Crack Down on Anti-Vaxxers," 7 Mar. 2019 Typically infectious disease outbreaks are declared over when there hasn’t been a new case for two full incubation periods of the particular disease. — Helen Branswell, STAT, "Health officials cautiously optimistic dangerous Ebola outbreak is over," 21 June 2018 One example is the terror management system, which psychologists use to explain how people manage their reactions to terrifying events such as natural disasters, infectious disease outbreaks or social threats from outsiders. — Wesley Wildman, Smithsonian, "How Computer Scientists Model the Role of Religion in Society," 11 June 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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More Definitions for infectious

infectious

adjective

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

See the full definition for infectious in the English Language Learners Dictionary

infectious

adjective

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

infectious

adjective

in·fec·tious | \ in-ˈfek-shəs

Medical Definition of infectious

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