infectious

UK: /ɪnˈfɛkʃəs/ US: /ɪnˈfɛkʃəs/ ,(in fek′shəs)

WordReference Random House Learner's Dictionary of American English © 2026

in•fec•tious /ɪnˈfɛkʃəs/   adj. 

1. Pathology (of a disease) spread by infection, as from one person to another or from one part of the body to another:
   If you have an infectious disease you really should not go to work.
2. tending to spread quickly and to many people:
   infectious laughter.

in•fec•tious•ly, adv.: 

grinned infectiously.

in•fec•tious•ness, n. [uncountable] See -fec-.

WordReference Random House Unabridged Dictionary of American English © 2026

in•fec•tious  (in fek′shəs), adj. 

1. Pathology communicable by infection, as from one person to another or from one part of the body to another:
   infectious diseases.
2. Pathology 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. [Obs.] diseased.

• infect(ion) + -ious 1535–45

in•fec′tious•ly, adv. 
in•fec′tious•ness, n. 
Synonyms

• 1. . catching. See contagious. 

Collins Concise English Dictionary © HarperCollins Publishers::

infectious / 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

inˈfectiously adverb

inˈfectiousness noun