Lethal Diagnosis

Being diagnosed with an illness is a surefire method for making it suddenly become much worse. Characters who have suffered one or two mild symptoms—such as a cough, fatigue, moments of disorientation, etc. -- will visit a doctor or hospital and be told they have some kind of tragic disease, then over the next couple of weeks or sometimes even days, they'll turn into feeble, hacking wrecks who ramble meaninglessly to themselves all day.

May be a psychosomatic effect, or may just be the writers cranking the intensity of the symptoms up, now that the cause doesn't have to be kept secret from the audience.

Film

 * In the movie Brazil, our first view of Mrs. Terrain has her with a few bandages due to a "complication" with treatment to make her look younger. Throughout the course of the movie her condition worsens despite her doctor's insistence that she'll soon be up and about, and in one of the dream sequences we see her coffin, which turns out to contain nothing but bones and something that looks unpleasantly like aspic.
 * Walt in Gran Torino. Sure, he was Coughing Up Blood anyway, but one gathers he was suffering with that for a long time before he went to see the doctor about it. Walt is never seen to get any worse, however his prognosis is pretty grim, by the way he suddenly tries to reach out to his 'good-for-nothing' son. Oddly, one never sees him deteriorate beyond that gruesome cough, from which he always quickly recovers.
 * Finding Neverland - where the reason for Sylvia's coughs is not revealed until late because she kept refusing to get treatment or act like anything was going wrong.

Live Action TV

 * Mark's dad in ER. And then, eventually, Mark himself, though in a much slower form.
 * Joyce Summers in Buffy the Vampire Slayer, who went from mild headaches to violent dementia in just a few episodes.
 * Can be Truth in Television. Plenty of people never even manage to get the diagnosis before they lose it.
 * A seemingly-healthy character in Scrubs comes in to the hospital because of a health scare on the news. After he coughs heavily they perform a chest x-ray and it is revealed he has lung cancer and dies within a few days.
 * They at least Lampshaded this when one of the doctors questions how he could even be walking around with a disease that bad.
 * Subverted, however, by Ben Sullivan (portrayed by Brendon Fraser)--Dr. Cox's best friend and ex-brother-in-law. In the first episode he was in, he was diagnosed with leukemia. He really didn't show any symptoms beyond an inability for his blood to congeal. In the third episode he's in, a year after his first appearance, he appears healthy but hasn't been seeing his oncologist for some time. Dr. Cox insists he get a workup and restart his treatments, but he dies twenty minutes after Dr. Cox goes off to run some errands for his son's birthday party.
 * Exception: In the first few minutes of the new Battlestar Galactica miniseries president-to-be Laura Roslin is diagnosed with terminal cancer, but the character proceeds to play a major role in the following TV serial without all the stereotypical signs of disease (and, on the whole, survives a lot longer than most who befall TV illness).
 * Every episode of House. Every time the team comes up with a theory, the patient's condition instantly escalates to point of unerring predictability.
 * That's almost entirely to do with the fact that the diagnosis is always wrong until the end of the show when they finally figure it out. Considering House's radical treatments, the diagnosis itself actually can make them worse. The number of times they give immunosuppressant steroids to people who end up having infections thinking they have some autoimmune disorder is amazing.
 * This is Played With in one episode where a woman comes in with a mild cough, which almost immidiately is revealed to be a particularly nasty form of lung cancer. Instead of informing the woman of her diagnosis, Cameron insistently denies the results, trying to come up with a less serious explanation. When she finally gives in and informs the woman that she only has weeks to live, she repeats that she just has a cough - and the viewer never sees her again.
 * Subverted in an episode of Gilligan's Island where everyone mistakes a harmless bug that landed on Gilligan for a bug with deadly venom. He shows all of the symptoms to everyone else, but there's a logical explanation: for example: Gilligan was just eating a bunch of bananas, so when Ginger offered him pie, he didn't want any. Everybody else thought this was a lack of appetite, one of the symptoms.
 * Star Trek: Deep Space Nine: Odo has been the carrier for the disease destroying his people for a long time without effect. Once this information is revealed, he's deteriorating rapidly within only a few episodes. This is handwaved in-verse with the explanation that the disease gains strength with increased use of shapeshifting and the excessive amount of shapeshifting Odo has had to do over a short space of time has massively escalated his condition. Still, it's only excessive shapeshifting by his standards and it wasn't like he didn't shapeshift prior to them... and it still affects him faster than it affects the rest of his people who shapeshift much more frequently on a long-term basis than he does.

Western Animation

 * In the Ed, Edd 'n' Eddy episode "A Case of Ed", Edd becomes convinced he has the rare (and presumably deadly) Lackadaisycathro Disease, and almost immediately becomes pallid and feverish.
 * Claude Cat in the Looney Tunes short "The Hypochondri-cat" was such a hypochondriac that even mentioning to him that he looked a little green would cause him to instantly turn that color. This reaction was played strictly for humor.
 * Parodied in South Park; in "Bloody Mary", upon being told that his alcoholism is a disease, Stan's father shaves his head, confines himself to a wheelchair, and speaks exclusively in a harsh whisper.
 * In one episode of Hey Arnold!, Helga is bit by a monkey and notices some redness in the area. She looks up a book on archaic diseases and becomes convinced that she has contracted a terminal, monkey-borne disease. Other symptoms include sweaty hands (which she develops out of fear of death), irritability (practically her defining character trait), and loss of appetite (impending death tends to make one less apt to enjoy a meal).

Real Life

 * In a related example from Real Life, Reader's Digest produced a Family Medical Guide in the 1970s that went through several editions. One section of the book included symptom flowcharts, and it seemed no matter how one followed the chart the answer was always some variation of "Get to a doctor right away." There was no such thing as "Your headache may be due to stress" or any other answer that didn't assume the absolute worst about a minor symptom.
 * Web MD has a similar problem. For virtually any symptom you enter, you can be nearly certain that cancer or syphilis will show up in your list of possible diagnoses.
 * One has to bring up the old Mark Twain chestnut here: "Be wary of reading health books. You may die of a misprint."
 * The UK's NHS Direct helpline occasionally tosses this up - although patients are told not to use it for anything that feels really unpleasant. Unfortunately, because the people on the other end can't see the patient, they frequently err on the side of caution and send people to A&E for minor problems.
 * In fact in Real Life AIDS develops faster on patients after being diagnosed.
 * This may be because AIDS is just a diagnoses of the progression of HIV, though it does fit the trope.
 * Pancreatic cancer often presents very mild and nondescript symptoms at first and is almost always rapidly fatal once it has progressed enough to be diagnosed.
 * Several cancers have a similar course, based on how late in the game symptoms arrive, but even pancreatic cancer has a non-zero survivability. The true kings of this trope are the prion diseases. Prions are renegade proteins that usually arise spontaneously in the brain, then transform other copies of the protein in the "bad" form. Symptoms usually only show up once a patient has a year of agony and increasing dementia to look forward to. The survivability is zero and there is nothing to slow the progression (even HIV can be treated). Neither Alzheimer's Dementia nor Huntington's has so rapid and terrible a progression. Prion diseases are mercifully rare, but given their gravity, fears of "Mad Cow" were not entirely groundless.
 * Lewis Black once commented in one of his stand-up routines that while on a plane flight, he read a magazine article on diabetes. "And when we landed, I had diabetes!"