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

The pandemic, which swept around the world in 2020, severely strained health systems and disrupted vaccination campaigns, resulting in a resurgence of infectious diseases such as measles and polio.

From Barron's • Apr. 24, 2026

The agency helped countries develop early warning systems to ensure that infectious disease outbreaks were rapidly detected and contained before they risked spreading to our borders.

From Slate • Apr. 23, 2026

Our public health system isn’t ready for these infectious diseases.

From Salon • Apr. 22, 2026

“There are pockets of vulnerability, like in communities, that can really lead to outbreaks going wild,” said Dr. Peter Chin-Hong, a UC San Francisco infectious diseases expert.

From Los Angeles Times • Apr. 16, 2026

The one-sidedness of that exchange of germs becomes even more striking when we recall that large, dense human populations are a prerequisite for the evolution of our crowd infectious diseases.

From "Guns, Germs, and Steel: The Fates of Human Societies" by Jared M. Diamond

Related Words

• contagious
• toxic
• virulent