infectious
Americanadjective
-
communicable by infection, as from one person to another or from one part of the body to another.
infectious diseases.
- Synonyms:
- catching
-
causing or communicating infection.
-
tending to spread from one to another.
infectious laughter.
-
Law. capable of contaminating with illegality; exposing to seizure or forfeiture.
-
Obsolete. diseased.
adjective
-
(of a disease) capable of being transmitted Compare contagious
-
(of a disease) caused by microorganisms, such as bacteria, viruses, or protozoa
-
causing or transmitting infection
-
tending or apt to spread, as from one person to another
infectious mirth
-
international law
-
tainting or capable of tainting with illegality
-
rendering liable to seizure or forfeiture
-
Synonym Usage
See contagious.
Other Word Forms
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.
Vocabulary lists containing infectious
"Romeo and Juliet" by William Shakespeare, Act V
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National Nurses Week: Common Medical Terms
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An American Plague
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
“That’s infectious throughout the whole secondary, and the whole defense.”
From Los Angeles Times ● Sep. 8, 2026
Online, some infectious disease doctors are critical of the details shared by the coroner because they say measles can cause the spleen to enlarge.
From The Wall Street Journal ● Sep. 4, 2026
He was not a dietitian or an infectious disease physician.
From Science Daily ● Aug. 27, 2026
Derek Mitchell, infused with infectious positivity, is looking to convert a former sugar refinery next to an Aldi car park into a thriving hub that will teach and enthuse young people.
From BBC ● Aug. 21, 2026
A few years back, Farmer had talked him into training as an infectious disease specialist.
From "Mountains Beyond Mountains" by Tracy Kidder and Michael French
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
