What does flu mean?
Flu is the common abbreviation for influenza, an acute and highly contagious viral infection of the respiratory tract caused by influenza viruses. Typical symptoms come on suddenly and include fever, chills, headache, cough, sore throat, and severe muscle aches and exhaustion — generally more intense than the sniffles and mild sore throat of a common cold. The word derives from the Italian term 'influenza', meaning 'influence', reflecting the old belief that epidemics arose under the influence of celestial bodies. Flu is central to everyday health vocabulary: people speak of catching the flu, getting a flu shot before flu season, or being out sick with the flu for days. A related informal phrase, 'stomach flu', describes vomiting-and-diarrhoea illnesses (gastroenteritis) that are not caused by influenza viruses at all. Because seasonal flu can be serious — even deadly for the elderly and vulnerable — the word carries real weight in public-health language, appearing constantly in news reports, vaccination campaigns, and workplace conversation.
nounAn infectious viral illness, short for influenza, that affects the respiratory system and causes fever, aching muscles, fatigue, and coughing.
- Influenza, the acute viral respiratory illness marked by fever, chills, muscle aches, and fatigue.
- Informally, any bout of similar flu-like illness, as in 'stomach flu' (viral gastroenteritis), which is not caused by influenza viruses.
"She was laid up with the flu for the entire first week of January."
"Half the office called in sick after the flu swept through the building."
"He got his flu shot in October and made it through the winter unscathed."
Flu is typically treated as a mass noun; a plural form is rare and generally unnecessary, though 'flus' occasionally appears informally when distinguishing separate outbreaks or strains.
"Different strains produced several distinct waves of flu that winter."
'Flu' is just an abbreviation of 'influenza' — an Italian word literally meaning 'influence', because people once blamed the illness on the influence of the stars.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of flu
Flu originated as an abbreviated form of 'influenza', borrowed from Italian during the eighteenth century when the disease's cause was unknown. The Italian term literally means 'influence', since outbreaks were popularly attributed to the influence of the stars or atmospheric conditions. It belongs to the same Latin root family as 'influence', 'influx', and 'confluence', all stemming from Latin 'influere' ('to flow in'). The clipped form 'flu' became standard in English by the mid-nineteenth century and has since almost entirely displaced the longer word in everyday speech.
How flu is actually used
Informal but universally understood abbreviation of 'influenza'; it appears far more often in everyday speech than the full form. Note that 'stomach flu' refers to gastroenteritis, which is not actually influenza. In casual use, 'the flu' is standard rather than bare 'flu'.
Easily confused with flu
The common cold is a milder respiratory illness usually causing sneezing and a runny nose without the high fever and severe body aches typical of flu.
A flue is a duct or pipe that vents smoke or gases from a furnace or fireplace — unrelated in meaning despite the near-identical sound.