infectious

American  

[in-fek-shuhs] / ɪnˈfɛk ʃəs /

adjective

1. communicable by infection, as from one person to another or from one part of the body to another.

   infectious diseases.

   Synonyms:

   catching

2. causing or communicating infection.

3. tending to spread from one to another.

   infectious laughter.

4. Law. capable of contaminating with illegality; exposing to seizure or forfeiture.

5. Obsolete. diseased.

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infectious British  

/ ɪnˈfɛkʃəs /

adjective

1. (of a disease) capable of being transmitted Compare contagious

2. (of a disease) caused by microorganisms, such as bacteria, viruses, or protozoa

3. causing or transmitting infection

4. tending or apt to spread, as from one person to another

   infectious mirth

5. international law

   1. tainting or capable of tainting with illegality

   2. rendering liable to seizure or forfeiture

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infectious Scientific  

/ ĭn-fĕk′shəs /

1. Capable of causing infection.

2. See Note at contagious

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Related Words

See contagious.

Other Word Forms

• infectiously adverb
• infectiousness noun
• noninfectious adjective
• noninfectiously adverb
• noninfectiousness noun
• uninfectious adjective
• uninfectiously adverb
• uninfectiousness noun

Etymology

Origin of infectious

First recorded in 1535–45; infect(ion) + -ious

Explanation

Infectious means able to be spread by infection, like a disease that spreads from one person to another. You'll be uncomfortable shaking Uncle Marvin's hand if you're worried that warts are infectious. Something that's infectious spreads easily, like infectious cold germs that might be left on a door handle — anyone who touches it might get sick. If your mother tells you the cut on your knee looks infectious, you'd better go to the doctor. Some infectious things are good — like infectious laughter that gets the whole class giggling, or your friend's infectious good mood that makes you forget your cares.

Example Sentences

"Joy, abundance of spirit, infectious laughter. A titan of a ginger haired man."

From BBC • Apr. 8, 2026

Dr. Amesh Adalja, an infectious disease expert and senior scholar at Johns Hopkins Center for Health Security, told Salon he is not hopeful the outbreaks can be contained at this point.

From Salon • Apr. 8, 2026

Hsiao plans to explore how these dietary changes influence the human gut microbiome and whether they could also help defend against other infectious bacteria.

From Science Daily • Apr. 7, 2026

After hours cooling his heels, Kornev is then informed that Stepniak is ill with an infectious disease — come back another time.

From Los Angeles Times • Mar. 26, 2026

Europeans may not have known about microbes, but they thoroughly understood infectious disease.

From "1491" by Charles C. Mann

Related Words

• contagious
• toxic
• virulent