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

There were no antibiotics to fight infectious diseases, no reliable antiseptics or anesthetics for surgeries.

From The Wall Street Journal • Apr. 14, 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

Nacua became known for his infectious enthusiasm and ability to catch almost anything Rams MVP quarterback Matthew Stafford threw his way.

From Los Angeles Times • Apr. 2, 2026

While it is not as infectious as illnesses like flu or Covid, it can take a turn for the worse "really quickly", he added.

From BBC • Mar. 17, 2026

If I so much as sneezed, she would rattle off a list of infectious diseases that would have me in the grave by nightfall.

From "Between Shades of Gray" by Ruta Sepetys

Related Words

• contagious
• toxic
• virulent