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

One of the biggest challenges in outbreak response, particularly for emerging infectious diseases, is the availability of reliable diagnostic assays that can quickly and accurately determine infection status. R. Daniel Bressler, Vox, "“Designer bugs”: how the next pandemic might come from a lab," 6 Dec. 2018 For example, in an impoverished country with a high risk of infectious disease and death in childbirth, the few wealthy citizens can avoid these risks through expensive healthcare. Cathleen O'grady, Ars Technica, "Genetics play less of a role in lifespan than we thought," 8 Nov. 2018 The good news, says Samuel Dominguez, a pediatric infectious disease doctor at the hospital, is that most of the children with enterovirus A71 have already recovered. Sumathi Reddy, WSJ, "Another Burst of Polio-Like Cases in Children Alarms Doctors," 5 Nov. 2018 AP Scott Dowell, an infectious disease doctor, has helped respond to more than a dozen Ebola outbreaks in the last two decades. Julia Belluz, Vox, "Ebola showed up in a war zone. It’s not going well.," 11 Oct. 2018 On one side are the majority of scientists and doctors, including an organization of more than 9,000 infectious disease doctors called the Infectious Diseases Society of America (IDSA). Rafal Tokarz,, "The Lyme Disease Complication You Don't Know About—But Should," 31 May 2018 The year 2004 was a simpler time to be an infectious disease doctor in the US. Megan Molteni, WIRED, "Insect-Borne Diseases Have Tripled. Here's Why.," 2 May 2018 Two infectious disease doctors study the lacerations on his face. Mark Shanahan,, "Was my son’s frightening injury an owl attack?," 28 Mar. 2018 Luckily, an infectious disease doctor had been on call at the hospital. Brianne Tracy,, "Toxic Shock Syndrome Survivor Lauren Wasser Explains Why She Was 'Ready' to Amputate Her Second Leg," 14 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

9 Dec 2018

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The first known use of infectious was in 1534

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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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