infectious
Americanadjective
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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.
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Law. capable of contaminating with illegality; exposing to seizure or forfeiture.
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Obsolete. diseased.
adjective
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(of a disease) capable of being transmitted Compare contagious
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(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
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tainting or capable of tainting with illegality
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rendering liable to seizure or forfeiture
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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.
“Public-health policies that single out non-U.S. citizens won’t prevent viruses from crossing our borders,” Jeanne Marrazzo, CEO of the Infectious Diseases Society of America, said in a statement.
From MarketWatch • May 18, 2026
Infectious disease experts break down the potential risk.
From Los Angeles Times • May 14, 2026
Army Medical Research Institute of Infectious Diseases, adding that the project is part of its larger initiative to tackle emerging infectious diseases.
From Barron's • May 11, 2026
Epidemiologist Eduardo López, head of the Department of Medicine and Infectious Diseases at the Ricardo Gutiérrez Children's Hospital in Buenos Aires, said further investigation in the province is necessary.
From BBC • May 10, 2026
Dr. William Currie’s sixty-four-page A Description of the Malignant, Infectious Fever Prevailing at Present in Philadelphia appeared at the beginning of September, when the fever was confined to a few streets and alleys.
From "An American Plague: The True and Terrifying Story of the Yellow Fever Epidemic of 1793" by Jim Murphy
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.
