infectious

adjective

in·​fec·​tious in-ˈfek-shəs 

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.

Synonyms of infectious

• spreading
• catching

Examples of infectious in a Sentence

viruses and other infectious agents

She has an infectious grin.

Recent Examples on the Web

Two centuries later, Louis Pasteur and Robert Koch helped establish germ theory, which holds that many diseases are caused by infectious agents that multiply in the body. — Jason Liebowitz, New Yorker, 25 Aug. 2026

Parton wrote the movie’s infectious theme song — which earned her the first of two Oscar nominations — while on the set between shots. — Deborah Wilker, HollywoodReporter, 25 Aug. 2026

Measles can spread when an infected person breathes, coughs or sneezes, and the virus remains infectious in the air and on surfaces for up to two hours after the person with infection leaves the area. — Jamie Gumbrecht, CNN Money, 25 Aug. 2026

On a recent episode of Therapuss, all Jake Shane needed to say was the word Boston and the episode’s guest, singer Role Model, began to croon Lefty’s infectious chorus. — Abigail Sylvor Greenberg, Vanity Fair, 21 Aug. 2026

Word History

Etymology

infecti- (in infection) + -ous

First Known Use

1534, in the meaning defined at sense 1a

Phrases Containing infectious

• infectious disease
• infectious hepatitis
• infectious mononucleosis

Rhymes for infectious

• noninfectious

Kids Definition

infectious

adjective

in·​fec·​tious in-ˈfek-shəs 

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 

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