TLDR:
Using the definite article before names of ailments was more common
historically than today. Even so, people still say the measles and the mumps about as often
as they say the flu, and you sometimes come upon others
like the gout, the cancer, and the rheumatism, especially in dialogue meant to represent older or more rustic speech.
The OED records that this
“ᴛʜᴇ-disease” version began over a thousand years ago, and
notes that both styles coëxist today. A secondary and sometimes more humorous subsense has since arisen in expressions like the blues, the creeps, and the jitters.
The Historical Record
Once upon a time, one used to speak of the influenza, so it is only natural that this would have become shortened up to the flu.
Many diseases were once commonly used with the definite article preceding their names in English over the past thousand years. Some still get talked about that way, often with both styles existing side by side in Present Day English. Jump to the section Free Samples, Often Funny towards the end of this post for copious examples of these, some with links.
The OED talks about this particular use of the article the in its two related subsenses for sense 8, the one technical and the other risible, with the text set in bold my own markup:
8a. With the names of diseases, ailments, etc.
Still in common use side by side with forms without the definite article.
8b. With colloq. or humorous names of afflictions, as the blues, collywobbles, creeps, D.T.’s, habdabs, heebie-jeebies, jitters, etc., q.v. Hence in analogous nonce-expressions.
The earliest citation provided is from Old English in 1000 ᴀ.ᴅ.; later citations include the cold, the croup, the fever, the stone, the German measles. So influenza is hardly unique.
In scientific journals you will see often see diseases referred to as the X virus, such as with influenza or smallpox or rubella, but when you drop the “virus” part, you also now normally drop the article.
But first before we talk about where we are now, let’s remember where we came from by looking at some of what Mitch would call arthrous uses. That is, those with the article before them.
That Was Then
So using the in front of diseases, ailments, and other assorted afflictions used to be perfectly normal. For example, in Volume 1 of The Baltimore Medical and Philosophical Lycæum from 1811 we find this reference:
I mention this more particularly, because in the influenza of 1790, Dr. Rush remarks that in several cases blood-letting produced alarming results.
And in The Obstetrical Journal of Great Britain and Ireland of 1875 we find this:
- [...] because the croup and the measles appeared at the same time: [...]
And in Famine in Tudor and Stuart England published in 1975 we find a reprint from an account originally from a couple centuries earlier reading:
- At times these and other afflictions came and also the bloody flux which put people in such a plight that countless died of it.
And here from 1831 in The Journal of Health with both the catarrh and the influenza as was then the common use:
- In proffering a few remarks on the epidemic catarrh, or the influenza, which has of late prevailed over so great an extent of country, [...]
This Is Now
When the Black Death scourged Europe, it was commonly called the Great Plague or the awful pest, with the latter giving rise to the organism we now know to have caused it, Yersinia pestis.
Today you would be more likely to hear someone talk about plague than to hear them talk about the plague. From the plague-inpested prairie-dog towns of my backyard in Boulder County, Colorado comes this warning:
- Plague is a bacterial disease transmitted by the fleas of rodents and is widespread in the western United States. Plague commonly infects prairie dogs and squirrels that can be found in parks and residential areas within Boulder County. Household pets, such as dogs and especially cats, can either get plague or carry infected fleas home to their owners.
This sounds perfectly natural here:
What killed off the prairie dog colony? Did they poison them?
No, it was the plague again.
Free Samples, Often Funny
The secondary, “humorous” use of the that the OED mentions is now commonly associated with references to the sufferer’s symptoms, such as in many of these:
Some of those come off as perfectly normal casual expressions, while others are mildly archaizing—a bit quaint as Janus mentions.
Summary
Saying the flu is a holdover from older styles of talking about illnesses and other physical troubles; people used to get the influenza. This practice was more common formerly than in Present Day English, where the habit—but not the rule—is to drop the article for most of these.
The casual terms amongst those listed above do seem to retain their article more than the official disease names. These euphemisms may be more noticeable in some regions an registers than in others.
Notes
Credit to Janus for suggesting the article usage might sometimes come off as a bit precious, and to Alan for suggesting the OED reference. Credit to Mitch for arthrous.
