There are all manners of terrible diseases out there, deadly and incapacitating, annoying and disfiguring, and even a few truly horrifying fates worse than death.
But in the world of fiction (and according to popular belief, in real life as well), one disease stands tall and terrible above the rest- a disease so deadly, so silent, so inevitable, so evil, that its very name has become synonymous with death. A disease whose mere mention provokes feelings of foreboding terror, disgust and paranoia in modern humans: the dreaded Malignant Neoplasm, or in layman's terms, Cancer.
There are countless types of cancer, but all share the same defining nature: in a cancer, the reproduction of cells goes out of control. Instead of dividing and multiplying in an orderly fashion, as they should, the cells multiply chaotically and violently without stopping, invading nearby parts of the body, and sometimes even spreading further through the bloodstream or lymphatic system. Ideally, cells have several countermeasures against such a destructive chain reaction starting, many emergency fall-back mechanisms meant to destroy the out of control cell before it can cause damage, but once in a while, those emergency killswitch mechanisms fail.
The causes of cancer are numerous, and can include anything from old age, to exposure to certain chemicals (such as the ones found in cigarette smoke), to exposure to great amounts of radiation (this association with the evils of nuclear power or weapons may be part of the reason for the fear of cancer underlying modern society's perception) to sheer bad luck. The truth is, that in the end, nobody can say for sure who will have cancer and when. Right now, your father may be developing cancer, or your girlfriend, or your dog. Even if they've never even been near Hiroshima or Chernobyl, even if they don't smoke, even if they live in a sterile bubble, they may still develop cancer. There is no way to tell for sure without checking, and even if they turn out not to have cancer right now, nothing can assure that they won't develop one tomorrow, or in three minutes.
Indeed, partially because of successful use of the aforementioned "failsafes" most of the human race is still alive, as 90% of the human race technically has cancer at any one time.
It could happen to anyone, any time, and they would die a slow, excruciating death as their bodies destroy themselves from the inside because of a tiny programming error in the coding of a single cell that prevented it from ever hearing the order to "stop multiplying". There would be few symptoms in the beginning, all of them easy to miss, but towards the end Body Horror will hit in full force as the tumors, black and slimy or bloated and red would become visible over the skin. Or perhaps, they never will, and all you would see is the patient writhing in agony for days before finally finding release in death. Or they could just go out with a single seizure, out of the blue...
Even if the cancer is discovered and medical treatment is available, the fact that a cancer is essentially an out of control aspect of the living body means that the only way to treat it effectively is to kill the growing body. Chemotherapy for cancer involves intentionally poisoning a person just enough not to kill them, but to (hopefully) kill the tumors growing in them. The side effects are horrific: over the course of the long, long treatment, chemotherapy and radiotherapy will cast the patient down into the dark depths of the Uncanny Valley, causing them to slowly and painfully lose anything they had of the image of a healthy human being. Hair will fall, nails will break, fat will disappear. They will vomit, and they will wither, and they will cry silently as they are transformed into pale, skinny mockeries of the human form not by a disease, but by the cure, helplessly watching this cruel world from behind lifeless, sleepy eyes.
At its core, a cancer is the perversion of the things we hold most sacred on the most fundamental level: reproduction, life, growth, fertility. The brightest and most wonderful things in nature twisted and mutated into something frightening, something repulsive and wrong by a mindless, faceless force that still somehow manages to seem outright malevolent. It is not surprising that just by mentioning the word "cancer", any instance biological horror may instantly become scarier, any scientific experiment more taboo, any associated magic or power evil.
Note that this trope isn't just for cases of cancer appearing in fiction: The Topic Of Cancer means that the word, term, or very concept of cancer is used specifically to evoke that semi-rational, paranoid terror and revulsion and creepiness that are associated with cancer, or with implications that cancer is somehow worse in some way than other diseases of conditions.
The Littlest Cancer Patient attempts to invoke this for emotional torque, often unsuccessfully.
The inversion is The Disease That Shall Not Be Named, where the work avoids saying cancer because it's so horrifying.
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Anti-Tobacco ads like to point out that tobacco use causes all sorts of horrible cancers as an effort to stop people from using Tobacco. Sometimes this comes with graphic images of cancer-ridden cadavers pulled from now dead smokers and/or actual dying cancer patients.
Anime and Manga
Used for karmic justice (and the compulsory Body Horror) in Franken Fran - a rich villain hires Fran to extend her life and tries to get her killed to steal her research (fortunately, Fran is Ambiguously Undead and can live through decapitation). Fran returns to the villain's mansion and finds her Showing Off the New Body - the experimental formula allowed her teleomers to regenerate indefinitely, making her cells immortal... Just as the first side effects start to appear, Fran calmy explains that the only cells not programmed to die of old age are cancer cells. She then walks away, leaving her client "immortal" - as a gibbering pile of semi-liquid flesh.
Elizabeta from the manga Gunslinger Girl was a young Russian ballerina who developed bone cancer in her teenage years because she spent a period of her childhood in an area of the Ukraine that was heavily affected by the Chernobyl incident. Due to the generally primitive state of public healthcare in ex-USSR areas, the cancer is not contained properly and by the time Elizabeta arrives in Italy for further treatment, the doctors decide that the best bet would be to amputate her leg before the cancer spreads. Realizing that she will never be able to dance again, Elizabeta tries to kill herself (it's not a very happy manga). This is remarkable because of all the other girls' backstories, this one is the second most detailed (by far) and the most often referenced (usually by people expressing pity over the poor, poor girl with the cancer), while the stories of girls who were nearly murdered, mutilated by car accidents, raped or born with horrific disfigurements get barely a line. The only story that gets more screen time than Elizabeta's is of the girl who was used as a living prop in snuff porn films.
Mitsuki Koyama, protagonist of the shojo manga Full Moon o Sagashite, has a cancer in her throat played for full terror-tragedy value. In her particular case, her dream is to become an idol singer, but the only way to remove the cancer by the point the story begins involves a surgery that would render her permanently mute.
The symptoms of the Apocalypse Virus from Guilty Crown are explicitly referred to as "cancerous", and constant comparisons to cancer are made. Needless to say, it is quite horrible.
Used as Fate Worse Than Death in one version of Spider-Man - It turns out Eddie Brock has cancer which, through hormonal imbalance, causes fits of rage, ruining his life. The symptoms also attract the symbiote to him. The symbiote wants to take over Peter but ends up attached to Brock and unable to switch hosts again. It has the power to stop the cancer from spreading but can't afford to cure it as it relies on it for food - this leaves Brock superpowered, angry and in constant pain - for the rest of his life.
A minor but very creepy Spider-Man villain Styx was at one point called "living cancer" - he was a victim of Playing with Syringes trying to find a cancer cure by way of Acquired Poison Immunity - by exposing him to mutagens. Instead it gave him a power to make anything he touches wither and rot. The experience also twisted his mind - if his ability wasn't limited to reach, he would be an Omnicidal Maniac.
One Incredible Hulk story (probably an annual) had a wealthy cancer patient come up with a convoluted plan to get the Hulk's gamma-irradiated blood in the hopes that this would make his normal cells superpowered and able to overcome the cancer. Instead, the cancer cells were the ones that became super-powered, with nightmarish results. There was a callback to this years later when Hulk learned that one of his old friends was dying of AIDS and refused to donate his blood for fear something similar would happen.
Doctor Manhattan from Watchmen is extremely distraught to discover that the radiation emitted by his physical avatar gave his friends cancer. This was actually a ploy by Ozymandias, who gave them cancer himself
The drug "Mayfly" from Empowered gives you super intelligence, but will kill you within 24 hours from brain cancer (that is, assuming it doesn't just do that without giving you anything in return). Unless you manage to use your newfound genius in order to survive... as a horrible, grotesque mass of cancerous flesh.
Possibly the classic Marvel Comics example: the death of Captain Mar-Vell. In his case, it happened in an earlier story when he stopped a villain stealing a container of a deadly nerve gas and it leaked during the fight. Mar-Vell managed to reseal it and eventually was given an antidote in time while unconscious, but the gas' carcinogenic effects would later gave him fatal cancer later, albeit suppressed for years by his nega-bands until it mutated past their resistance and making impossible to treat without assistance from the Kree medical community, which was a non-starter for a traitor like himself.
Here's a fun little story for ya; a nice young man discovers he has the big C and, not wishing to drag it out, refuses chemo. Then he's offered the chance to cure his cancer by Weapon X, who implant a version of Wolverine's Healing Factor in him. Only the cancer and healing factor play nice together and their constant struggle winds up turning him into one handsome, crazy son of a hired gun. Ain't that a bitch?
A bit of an oddball example is in Tommy Wiseau's The Room, wherein the Love Interest's mom flat-out states that she has terminal breast cancer during one scene. This is promptly forgotten and never mentioned again.
One day, the main character of Phenomenon sees a flash of unearthly light, hears a strange sound, and falls unconscious. When he wakes up, he's got super-intelligence and even psychic powers. Turns out he's got a massive cancer in his brain that by complete coincidence happens to enhance certain functions by pushing against rarely activated parts of it.The flash of light was an epileptic hallucination brought upon by the cancer. Oh, and the main character is also dying.
Agent Smith: Human beings are a disease, a cancer of this planet. You are a plague, and we... are the cure.
The story "Metastasis" by Dan Simmons is built heavily around this trope. Apparently cancer is caused by a parasitic race that uses humans as breeding grounds. In effect each tumor is actually a larva that devours humans from the inside as it reproduces wildly until there is nothing left. The parents then come back and devour their offspring for nourishment occasionally leaving one or two to germinate into an adult.
Tuf Voyaging has a "cancer creature/living tumor" example in the "Meatbeast" that Haviland Tuf proposes as a temporary solution to S'uthlam's overpopulation induced food shortage.
Sam, the title character in Ways to Live Forever, has leukemia; the book also has a film adaptation.
Inverted The Fault in Our Stars, both of the main protagonists (any half of the supporting cast) have cancer. They all speak of their condition with a blase-ness that only terminal cancer patients can muster. And people who try to sentimentalize or go emotionally overboard over the condition are heartily mocked.
Scully almost died of alien-induced cancer. Other abductees were not as lucky.
In one episode the Monster of the Week was a man made entirely of cancerous tissue. He could regenerate the loss of his head and had to eat cancerous tissue removed from surgery patients (he worked in a hospital).
The villain of "Pusher," Robert Modell, is a man whose inoperable brain tumor came with incredible mind control abilities. With nothing else to live for and excruciating headaches, Modell hired himself out as a mercenary/hitman, using his powers to kill for huge profits. Funnily enough, it was written by the same man who went on to do Breaking Bad.
In medical tv shows, Scrubs for example, while cancer -is- often shown to be horrible, other types of diseases can be shown to be just as bad at times.
The series Breaking Bad revolves around the protagonist, Walter White, discovering during the pilot that he has terminal lung cancer. It is this discovery that kick-starts the action of the show.
An early monologue by Bruce McCullough on The Kids in the Hall had him apologizing to everyone for "causing all of that cancer". He says he did it because he "didn't realize it was such a horrifying disease" and he was "just on a roll".
In House, now that the oncologist, Wilson, himself has cancer, it's an opportunity for the writers to both show more of their relationship with House, and how firsthand knowledge of what cancer actually does to patients affects their reasoning when seeking treatment.
Downton Abbey: Early on in Series 3, she finds a lump on her breast and meets with Dr Clarkson for diagnosis. She asks Mrs Patmore to tell Carson that the lump is benign, but it is left unclear as to whether she tells him this so as not to cause a fuss and worry him.
Helen Givens from Justified. We learn that she is a cancer survivor during an argument with Arlo in Season 2.
Arlo: I hope you get cancer!
Helen: Already had it!
Arlo: I hope you get it again!
In Everwood, Rose Abbott is diagnosed with cancer. She survives, but she suffers horribly, going through chemo and operation procedures. Also her family are devastated with fear of losing her.
Call of Cthulhu supplement The Asylum and Other Tales, adventure "The Asylum". One of the drugs Dr. Freygan uses to create proto-shoggoths is called Cellular Accelerator. It can heal wounds quickly, but there's a chance it will cause cancer throughout the recipient's body. The victim will be reduced to a mass of suppurating flesh within 2-12 days and die 1-3 days later.
Amber Diceless Role Playing. If characters with Shape Shifting push themselves beyond their limits while suffering from exhaustion or starvation, they can suffer from Primal Chaos Cancer. Some of their cells run wild, consuming normal cells and multiplying quickly. The rogue cells will eventually attack vital organs, appear on the skin and eat the character alive, turning him into an amorphous blob.
In Promethean: The Created, the mysterious sixth Lineage are the "Zeka", Prometheans animated and fueled by radiation that have a lot of disturbing, creepy abilities. Among them are the ability to give other people regular, boring old cancer, but if you want to get freaky, you can give them a tumor with a mouth that can speak to them, or a mind control brain tumor, that is implied to cause endless excruciating pain and leave the victim a helpless puppet with full consciousness but no control over their actions. When Zeka use their powers, their "Disfigurement" usually involves huge cancerous masses growing from within them, boiling, bloating, and pulsating as their skin falls off and their eyes melt. Even when not showing their powers, most of them have signs of radiation poisoning or cancer, such as visible tumors on their faces. Then there are the Carcinoma, which is what you get when Zeka tries to make more of themselves and fail horribly: hideous monstrosities made completely out of cancer, with an alien, incomprehensible, malignant intelligence.
"Hunchback: The Lurching", a fan gameline for the New World of Darkness: As a drinking game, take a shot when the lump is described as "Cancerous", "Malignant" or "Tumorous". Just don't blame me when you lose your liver.
Wit: Perhaps one of the most brutal works on the subject.
In the original Civilization, developing a cure for cancer gave you one happy citizen in every single city, no strings attached.
The creepy biological macguffin from Oni, the Chrysalis, is repeatedly described as being like cancer.
The page quote comes from Mondo Medicals. Let's just say a certain character is... passionate about the Topic of Cancer.
By the time of Metal Gear Solid 4, Naomi Hunter, a beloved character who's been with Snake (and the player) all the way since the first game doesn't just have cancer, she's practically already biologically dead from cancer. It's so bad that mere seconds after she deactivates the nanomachines in her body fighting it, she dies.