English Reference
noun

flu

n.
US /ˈfɫu/ UK /flˈuː/ flu UK spelling flus

  1. 01 A2 Elementary Medicine

    a common illness that gives you a fever, a headache, and makes your muscles ache.

    an acute respiratory infection caused by influenza viruses. Uncountable in general usage; often takes the definite article 'the'.

    Simple

    I couldn't go to work because I had the flu and a high fever.

    In context

    Public health officials urged vulnerable populations to receive their vaccinations before the flu season reached its peak in mid-winter.

    Usage

    Usually used with the definite article 'the'.

    Pitfall
    I have a flu. I have the flu.

    In English, we almost always use 'the' with flu when talking about being sick.

    influenzagrippe
  2. 02 B1 Intermediate Informal Medicine

    an illness that makes you feel sick to your stomach, often causing vomiting.

    a colloquial term for gastroenteritis or similar digestive upsets, though medically distinct from influenza.

    Simple

    Half the kids in the class were out last week with a stomach flu.

    In context

    Although the patient described his symptoms as 'stomach flu', the physician suspected a bacterial infection from contaminated seafood.

    Teacher's tip

    Medical professionals prefer 'gastroenteritis' as 'flu' technically only refers to respiratory viruses.

    gastroenteritisstomach bug

Clipping of influenza.

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

This entry
Level A2 · Elementary
Frequency Frequent
Updated