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
Looking to grow your vocabulary? Check out this interactive, curated word list from our team of English language specialists at Vocabulary.com – one of over 17,000 lists we've built to help learners worldwide!
National Nurses Week: Common Medical Terms
Interested in learning more words like this one? Our team at Vocabulary.com has got you covered! You can review flashcards, quiz yourself, practice spelling, and more – and it's all completely free to use!
An American Plague
Want to remember this word for good? Start your learning journey today with our library of interactive, themed word lists built by the experts at Vocabulary.com – we'll help you make the most of your study time!
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.
Sometimes the energy of all that “na-na-nah”-ing is infectious enough to unify rivals, if only for a few measures of music.
From Salon ● Sep. 13, 2026
The findings suggest that longevity may depend in part on an immune system that remains highly effective against both infectious organisms and cancer.
From Science Daily ● Sep. 12, 2026
Some unpaid health workers are violating safety protocols by allowing grieving families to touch highly infectious corpses.
From The Wall Street Journal ● Sep. 8, 2026
“That’s infectious throughout the whole secondary, and the whole defense.”
From Los Angeles Times ● Sep. 8, 2026
Why did the rise of agriculture launch the evolution of our crowd infectious diseases?
From "Guns, Germs, and Steel: The Fates of Human Societies" by Jared M. Diamond
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.
