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

• spreading
• catching

Examples of infectious in a Sentence

viruses and other infectious agents

She has an infectious grin.

Recent Examples on the Web

Watching golf bring people happiness brought him happiness, and that infectious personality was felt by everyone who met him. —Ryan Coleman, Entertainment Weekly, 27 Oct. 2025

Later, Gonzalez’s infectious energy and in-your-face defense helped the Celtics climb back into the game after an ugly second quarter stuck them in a 64-44 halftime hole. —Zack Cox, Boston Herald, 25 Oct. 2025

Merchandising was a side effect of this infectious spread, an invitation and a demand to consume the trappings of the images that screens emitted. —Ian Bogost, The Atlantic, 23 Oct. 2025

In a story that is sometimes outweighed by its tragic end, Selena y Los Dinos manages to radiate joy by showing Selena as a human being, vibrating with energy, determination, and an infectious sense of humor. —Cat Cardenas, Rolling Stone, 21 Oct. 2025

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