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
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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.
For more than 50 years, with a few breaks in between, Squeeze has been making memorable tunes by marrying infectious pop melodies with sophisticated and literate lyrics, often about the complexity of relationships.
From Los Angeles Times ● Sep. 16, 2026
New World screwworm does not spread from one person to another the way an infectious disease such as COVID-19 does.
From Science Daily ● Sep. 15, 2026
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
Dr. Jessica Ericson, a pediatric infectious disease physician at Penn State Health Golisano Children’s Hospital, hadn’t seen a measles case in her 15 years in medicine until this June.
From The Wall Street Journal ● Sep. 4, 2026
Neither did the early farmers understand that feeding children with more porridge and less breast milk would weaken their immune system, and that permanent settlements would be hotbeds for infectious diseases.
From "Sapiens: A Brief History of Humankind" by Yuval Noah Harari
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.
