Soap Opera Disease

Zorc: I have a terminal disease!
Yami Bakura: But you can't die! What about our adopted daughter? Who's going to take care of her when you're gone?

Zorc: She also has a terminal disease!

—Zorc and Pals, Yu-Gi-Oh!: The Abridged Series.

her condition was a case of terminal moe, a dangerous illness which crops up in anime whenever they want to make a character more sympathetic through illness but don't want to actually attribute a real illness to them

—Gabriel Blessing comment on the real illnes behind Chiho condition

Common ailment found on the Soap Opera. It is notoriously difficult to diagnose, and often exhibits vague symptoms. Often fatal, but victim can linger on for a long, long time. Sometimes results in a lengthy coma which, upon the victim's recovery, may also manifest a radical change in appearance (see: The Other Darrin).

J. Michael Straczynski has mentioned one riff on this, in which a soap opera writer supposedly put a character into a coma with very specific and carefully-researched symptoms, for a plot that was only supposed to last a couple of episodes... and then realized that if the character's recovery was delayed, he could continue to crank out script after script without ever needing to worry about a pink slip, since nobody else on the writing team knew enough about the disease to write the character's recovery. The actress didn't mind either, since she got a paycheck day after day for a few minutes' laying absolutely still in bed. The producers were somewhat less pleased at this.

Of course, the anime trope of the Ill Girl suffers from the same plague of vague. A common mutation anywhere is the Incurable Cough of Death, a terminal illness with no symptoms of any kind besides coughing. More likely than not evolved from Victorian Novel Disease. The character may insist the illness is Definitely Just a Cold.

Not to be confused with Soap Opera Rapid Aging Syndrome, which isn't actually a disease so much as a Plot Hole.

Examples of Soap Opera Disease include:

Anime & Manga

At one point in Ranma ½, Master Happosai suffers from a case of this. Unfortunately, he gets better. Also, the teacher Hinako Ninomiya had a case when she was young. She gets better as well.
In Ghibli's "The Borrower Arrietty", the male protagonist Sho has a heart condition ever since he was a child. Just a short period of physical activity can cause him pain.

Comic Books

Miss Grundy gets diagnosed with this in the first issue of Life With Archie: The Married Life, and dies a few issues later as a result.

Film

Dancer In the Dark, a drama and musical film, focuses on a female protagonist who has a genetic disease that causes her vision to weaken as she gets older. Early on in the film, she is already clinically blind, and this serves as a big element of the plot.
Parodied in The Living Wake where the self-deluded main character is certain he will die of "a vague and grave disease" despite clearly being in perfect health. He ends the wake by stepping ceremoniously into a coffin where he instantly dies.
Love Story has been accused of playing this straight. (see Magazines, below.)

Live Action TV

Averted in non-American soaps, which are more concerned with realism. The closest thing seen on Home and Away is probably diabetes.
Appears in, of all things, the profiler procedural Criminal Minds. In an early episode, in a very dramatic scene, Hotch's son has had to go to the hospital to get "some tests" done and it turns out he has "a condition". This is promptly never mentioned again.
The protagonist in One Liter of Tears suffers from a harsher and much rarer kind of this.

Magazines

Mad Magazine referred to this as "Old Movie Disease" in their parody of the movie Love Story, along with the claim that it makes you more attractive so you can die a beautiful death.

Video Games

In Eternal Sonata, if you're able to use magic powers, it also means that you have vaguely defined illness that is eventually fatal, though it's hard to say exactly when.
Any Key Visual Arts game will have at least one Ill Girl that suffers from this.

Web Original

Inverted in the Paradise setting, in which humans are randomly, permanently Changed into Funny Animals by causes unknown. The change is Invisible to Normals, to whom the Changed will still appear to be his old human self. In order to prevent Changed from being injured by medical practitioners because of physiological differences the medics can't see, the Changed invented a fictitious real-world disease—-"Sleeping Sickness (Ivory Coast Variant)"-—and issued medical alert bracelets for it so that a Changed or Known physician could be alerted at need.

Western Animation

Mocked (as with many other soap opera tropes) in the All My Circuits segments of Futurama.
- "That was so terrible, I think you gave me cancer!"