Here's the situation:

I have a flu.


I have the flu.

I saw some lesson on website about the vocabulary of illnesses.

Illnesses that use the article -the:

but some on website say I have a flu. so, I'm confused about the article for flu.

Should I use the or a for flu?

Thank you in advance.

  • 2
    Native speakers catch the flu at the library and then they go to the doctor. It is idiomatic to use the definite article with the ubiquitous and the universal. Once they're well, they can take a trip to the beach.
    – TimR
    Jun 18, 2018 at 11:25

2 Answers 2


You can't use a because flu is an uncountable noun.

According to Cambridge Dictionary

flu noun [ U ]

a common infectious illness that causes fever and headache:

a flu virus
to catch/get/have (the) flu

U - means uncountable. You can have more or less flu but you can't have a flu or two flus. Also notice that the example given indicate the use of the.

More info about countable vs uncountable, here.

EDITED: userr2684291 pointed that The flu is used in AmE, while flu or occasionally the flu (somewhat old-fashioned) in BrE

  • @userr2684291 Edited to include your comment. Thanks!
    – RubioRic
    Jun 18, 2018 at 9:26

@RubioRic's answer is correct to a degree. Most people colloquially refer to the illness as "the flu", however as you can see from the dictionary quotation he cites you can refer to "a flu virus". It would be medically incorrect to say "the flu virus" unless you had already defined a specific virus that you were speaking about. It is too simplistic to say that "the" should go before "flu" and never "a".

There are many different strains of flu (or influenza to use its proper name) and these are caused by distinct influenza viruses. As these viruses are countable then these would be referred to as "a flu virus" or "an influenza virus".

It is also common to omit "the" altogether and simply say:

I have flu.

The British National Health Service page on flu uses both "the flu" and simply "flu" in the same article, but with a clear bias towards the latter:

"Flu is very infectious.."
"GPs don't recommend antibiotics for flu.."
"How to treat flu yourself"

It does contain one use of:

"The flu vaccine..."

but this is referring to the vaccine, not the flu.

And with specific new strains of flu being diagnosed in recent years these are sometimes referred to by their proper names, even outside the medical profession, such as H1N1 (commonly referred to as swine flu) or "bird flu". Interestingly these are never referred to as "the swine flu".

The use of "the" when referring to an illness is somewhat of a legacy and not always used. You don't hear people say "I have the headache" - we say "a headache" despite the fact we only have one head. Neither do you hear people speak of (relatively) recently diagnosed conditions this way, such as "the AIDS" (not correctly used to my knowledge). It tends to be restricted to diseases that were historically described that way for example the pox (an archaic term for syphilis that dates from late middle English but was still commonly used as late as the 1970s in British English), or even the plague.

  • 1
    i would say that that is because viruses are countable (virus noun [ C ]), and flu are a subset of them.
    – WendyG
    Jun 18, 2018 at 11:06
  • The word karkinos, Greek for 'crab', was first used to describe the disease by Hippocrates, so 'the 'flu' is newer than cancer ('influenza' came into English in the 18th century, but 'the flu' is a later anglicization). Lack of an article might be due to our word 'cancer' coming from the Latin; the few diseases that I can think of that do have an article tend not to be Latin terms but English coinings or anglicizations - 'the flu' vs 'influenza', 'the cramps' vs 'dysmenorrhea', 'the squits' vs 'diarrhoea', 'the bends' vs 'decompression sickness'. Jun 18, 2018 at 11:44
  • @PeteKirkham Well Pete, there are certainly those early names of conditions that we now believe were likely describing cancer. But we don't use those names. If we did, perhaps we would call cancer the crab. Unless you can cite an example of a modern diagnosis which we refer to using the, my point stands. But to make you happy I'll change "the cancer" to "the AIDS".
    – Astralbee
    Jun 18, 2018 at 12:28
  • 2
    Native AmEn speaker here, and I've never heard the phrase "I have flu" before. (Though just seeing in RubioRic's answer that that phrase is more common in BrEn.)
    – David K
    Jun 18, 2018 at 12:42
  • 1
    @DavidK I have added several of the quotations from the NHS website to my answer.
    – Astralbee
    Jun 18, 2018 at 12:53

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