English Reference
adjective

infectious

adj.
US /ˌɪnˈfɛkʃəs/ UK /ɪnfˈɛkʃəs/ in·fec·tious

  1. 01 B2 Upper Intermediate Technical Medicine

    describes a disease that can be passed from one person to another through germs or viruses.

    relating to a disease caused by a pathogen that enters the body and is capable of being transmitted to other hosts.

    Simple

    The doctor warned that the flu is highly infectious during the first few days.

    In context

    Public health officials monitored the outbreak closely, concerned by how infectious the new strain appeared to be in densely populated urban areas.

    contagiouscommunicabletransmissible
    non infectiousharmless
  2. 02 B2 Upper Intermediate

    describes a feeling or behavior, like a laugh or a smile, that quickly spreads to other people.

    tending to spread from person to person through social contact or emotional resonance.

    Simple

    Her infectious laugh soon had everyone in the room giggling along with her.

    In context

    The team captain's infectious enthusiasm served as a catalyst for the younger players, lifting their spirits despite the early deficit in the score.

  3. 03 C1 Advanced Informal

    describes something very exciting or catchy that makes people want to join in.

    evoking excitement or interest in a way that is easily picked up by an audience or group.

    Simple

    The band played a song with an infectious beat that got everyone dancing.

    In context

    The marketing campaign relied on an infectious energy that resonated with younger consumers, leading to a rapid increase in brand recognition.

    boringuninspiring

Usage infectious disease

They’re clearly over the moon – and it’s infectious.

The Guardian· 24 Aug 2026

As an infectious disease epidemiologist , I study the reasons for changes in population health risks around the world.

The Conversation· 21 Aug 2026

Borrowed from New Latin īnfectiōsus. By surface analysis, infect + -ious.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Level B2 · Upper Intermediate
Frequency Frequent
Updated