The Melding Plague in Alastair Reynolds' Revelation Space trilogy. It's harmless if you're a baseline human, but if you have any advanced technology (nanotechnology) in you, it will infect those and cause them to rapidly fail and go out of control. In Chasm City, a character mentions that if you have those implants in your head, your head will explode. And it can infect advanced buildings and vehicles as well. In Chasm City, inhabitants of high-tech buildings were trapped in the walls, visibly screaming in terror. The survivors of the disaster don't know if it's possible to revive them.
Also from the Revelation Space universe - Nightingale, a quite insane medical AI that creates artistic statements against war using living human beings as its medium.
Reynolds' short story "Diamond Dogs" has the main character being slowly and voluntarily turned from a human into a cybernetic dog like creature. Unfortunately, the doctor who did this took himself apart so he wouldn't have to undo his "greatest work".
In Jerome Bixby's "It's a Good Life", little Anthony (a Creepy Child of the first order) transforms obstreperous party guest Dan Hollis into... something-or-other unspeakably horrific, then wishes him "into the cornfield."
The main character in Treason by Orson Scott Card, Lanik Mueller, is a radical regenerative who grows extra genitalia, arms, heads, everything. He winds up growing a clone of himself and freaks out.
Said character is a member of a nation that has been genetically modified to heal quickly from virtually all wounds, and even regrow limbs. During puberty, it is common for them to grow extra body parts, which are subsequently removed. Occasionally, a mutation occurs in which the extra body parts, as well as many additional ones, continue to regrow, resulting in monstrosities (at one point the protagonist has at least five legs, four arms (and probably more), four ears, two noses, three sets of genitalia, and two hearts).
The Witches by Roald Dahl: In the chapter "Metamorphosis", the witches hold down the boy protagonist and feed him Mouse-Maker, which transforms him into a mouse permanently and painfully. The transformation isn't permanent in The Film of the Book, but the visuals make it even more disturbing.
They did the same thing to his friend, at least in the film, which he gets to watch in horror.
"The Abnormals" aka "The Discarded" abounds with it. And it's illustrated vividly in the Stephen Hawking Sci-Fi Masters TV adaptation of it. A bunch of people are cast off into space on an old spaceship after they were mutated by a virus. There are a couple of fish-men, though they don't appear in the TV episode. The leader, Sanswope, grew an extra head. Another sort-of leader has a hugely oversized hand and arm. There's a woman with porcupine quills and background character with one eye and a flattened head. There are people with tentacle arms or feet. But the worst one, at least onscreen, is big arm guy's girlfriend, whose torso skin is transparent and allows a view of her organs when she disrobes.
The War Against the Chtorr. In A Rage for Revenge, the leader of a cult that worships the alien invaders removes his clothes to reveal that his body is covered in 'worm fur', the neural symbionts that act as sense organs for the Chtorran worms. And in A Season for Slaughter, an expedition discovers that Chtorran cities are somehow capable of transforming the lifeforms within them— including captive humans.
"The Metamorphosis" by Franz Kafka. Average Guy wakes up as an unknown human-sized insect, unable to perform many of the actions natural for human. It gets even worse when his father throws some apples at him in anger and second one penetrates his body armor and stuck, rotting and eventually causing his death.
Another famous Czech writer, Jaroslav Hasek, did this frequently in his satirical short stories. For example, in "The Austrian Customs" a man is composed from scratch after the train wreck, using animal parts and artificial prosthetics. The story's premise is very similar to RoboCop (1987), including a scene where a person visits his own grave (a scene planned for but left out of the movie). The reason for its title? Austrian Customs at the time (1910's) forbade importation of pig meat and the man had an implanted pig liver.
The short story "Gray Matter" (appears in the collection Night Shift) is about a son watching his father slowly turns into a vaguely humanoid fungus creature due to drinking too much tainted beer. The concept might sound a little silly, but the descriptions of his gradual transformation certainly aren't.
This story is a homage to Arthur Machen's classic tale "The White Powder", a much more horrific and completely unfunny version of the same phenomenon.
In the short story "I Am the Doorway" (also in Night Shift), aliens inhibit the main character's body, and eyes appear on his fingertips.
In The Dark Half, villain Stark's body "loses cohesion" and he starts to decay and rot.
The Stand had a little of it. Captain Trips caused the victims to end up with blackened, swollen necks (leading to the nickname 'Tube Neck' in some areas)and lymph glands.
In "Cool Air", the learned doctor melts. He's actually been dead for years.
In the story he revised, "Two Black Bottles": A man disintegrates after his soul is released. It's described very graphically.
The forerunner to this was Edgar Allan Poe's "The Facts in the Case of M. Valdemar".
At the end of "The Thing On The Doorstep", the protagonist's friend appears on the eponymous doorstep with his mind trapped in the decomposing body of his dead wife.
"Under the Pyramids" (ghostwritten for Harry Houdini), aside from the inevitable Eldritch Abomination, suggests that a corrupt branch of the ancient Egyptian priesthood had made a practice of stitching together patchworks of human and animal corpses, in emulation of their animal-headed gods. Shadows of these mix-and-match mummies are seen moving across the walls, implying either that they're undead, or that the priests had scooped Dr. Frankenstein's breakthrough by millennia and brought their hybrids to life.
The Remade, criminals who are punished by having their bodies altered in horrific ways. There's also a Remade brothel.
Breaking Dawn, Book II. It'll make you miss the sparkly teenage romance.
In fact, Spoony argues that the only person who could direct a faithful screen adaptation of it is David Cronenberg (see Film).
In Discworld, Angua's breed of werewolf can look like either a normal human or a normal wolf, but the few seconds of transition between the two is so horrific that she never lets anyone see it if she can help it. (This being a novel series, we have to Take Their Word For It, and we're glad to do so.)
There's also the thankfully brief description of Cosmo Lavish's hand, which has become rotten and gangrenous from wearing a too-small signet ring near the end of Making Money.
The Mistborn series by Brandon Sanderson includes some of this; the process of Hemalurgy, specifically, involves killing a person in order to transfer his or her powers (and/or soul) to another person by piercing them with a piece of metal, usually a large spike. The Lord Ruler uses this to create his Steel Inquisitors, which have gigantic metal spikes shoved into their eyes, among other places; Koloss, who the Hemalurgy mutates into monstrous, inhuman war machines whose skin is replaced by that of a different Koloss and which never grows larger (thus a newly-created Koloss will have baggy skin that would fall off if it weren't fastened on with spikes, and the oldest and largest Koloss have skin that has stretched so far that it's torn off of them); and kandra, shapeshifters which absorb the bones of dead creatures to support their bodies, which otherwise are just blobs of flesh.
Scott Smith's novel The Ruins: When a character thinks that the man-eating vines are growing underneath his skin and begins obsessively cutting himself open to try and get rid of the tendrils. The worst part? HE'S NOT HALLUCINATING.
No, the WORST part is that they did that scene in the movie too, only with a weeping, frantic young woman as the victim. And it's clear that she's torturing herself for nothing, because the vines are visibly creeping all over her body, guaranteeing she'll die even if she flays herself completely.
In the later books of The Wheel of Time, there is body horror all over the place when the Dark One's prison weakens and he can begin to "touch the world". What's worse is that it always strikes out of NOWHERE. One man is feverish and then suddenly bursts into flame and slowly burns to death. Some people are found as charred corpses in their beds. Another guy explodes into a swarm of bugs. There are several more as well.
In one Animorphs book, when the Animorphs start demorphing just as the two hour time-limit is reached, there's a very real possibility of them being trapped halfway through and being human/animal hybrids forever.
The fact that they occasionally get the science wrong (notably, characters' knees reverse themselves when transforming into wolves, which isn't how a wolf's leg works at all—what the author takes for knees are actually ankles) just makes it worse.
Morphing explicitly doesn't work "logically" or consistently — thus Cassie, with her talent for it, is able to mimic a centaur at the half-way point between horse and human, despite neither form having six limbs.
Morphing in general is described as pure Body Horror: the body mutates and changes in a random, uncontrollable sequence, in ways that look disgusting and horrifying (one morph involved a character's bones briefly becoming visible before flesh flowed over them. Another had dragonfly eyes suddenly appear on an otherwise normal human head). It isn't painful, but it looks like it should be. Only estreen, or people with a natural "talent" for morphing (like Cassie), can avert the horror aspects and make the process look beautiful.
Also, whenever they morph the extra mass is sent to Zero-Space a blank void that ships travel in for faster than light speeds. At one point a passing ship pulls the mass with it, causing the Animorphs to "snap" into Zero-Space...one piece at a time. While this wasn't painful either, it was horrifying to witness. The Animorphs aren't too interested in hearing about how they rewrote the Andalite science textbooks.
A series by the same author, Remnants, has a character who has his skin removed and replaced in small squares with a clear substance while his father is forced to watch. Like the morphing above, though, it doesn't hurt the affected person; it just looks ghastly.
Without speaking Winston lifted his shirt, and Jamie had to hold back a scream. A burst of glowing light poured out like blood, and it looked as though the middle of his chest had been dug out and replaced with hot coals. The skin around it was smoking and blackened; there was a smell of cooking meat...
Another of the Matter Manipulator's subjects is Tallow of the Freakshow, so named for the fact that his flesh is constantly melting like candlewax, and every so often, he has to reabsorb the pieces back into himself. The sign beneath his tank reads "This is Tallow: his every living movement is hellish," and how true.
But no other Goosebumps book was more pornographic about Body Horror than Chicken Chicken (number 53 of the original series), about two farm kids, Crystal and Cole, who get cursed into turning into chickens after knocking over a strange woman in black. Almost every chapter of the book detailed how the feathers were growing on the kids' skins, how their mouths turned to beaks, how they clucked every time they spoke and no one seemed to care about their transformation. See this blog review for details.
Goosebumps loves this sort of story, and another such book was adapted as a television episode. It's about a boy slowly changing into a dog while nobody around him seems to care (the twist is that he was always a dog, and an experiment to turn him human is simply wearing off). It's all fun and games until you watch it unfolding as a grotesque metaphor for puberty, with the boy shaving off the bizarre hair sprouting all over his body and trying to hold onto his dwindling humanity.
What about "I Live in Your Basement?" A girl pulls herself inside out through her mouth.
Writing as Jack Yeovil for Games Workshop, Newman's Dark Future titles had people being transformed into half-human, half-reptile hybrids by shadowy GenTech; complete with bone shifts, skin flaking off to become scales and extra teeth erupting from the jawline. The Demon Download series also featured The Path of Joseph, whose adherents were lucky enough to experience the joys of being slowly mutated into Barbie-esque carbon copies of Donny and Marie Walton. Although the transformation process itself is never directly described, the Waltonites are all identical, plastic-parted clones who spew a mish-mash of sanctimonious prayers and Sears Catalogue advertisements and lack nipples, genitals and individual toes. They don't bleed, either.
Dean Koontz's novel The Bad Place features a main character who initially teleports involuntarily. However, the more he jumps, the more he loses focus, with parts of his clothes being patchworked and actually ending up in his skin. It's when he finds half a cockroach has been melded into his shoe that the horror goes into overdrive, ruminating on whether parts of the roach have ended up in his brain or organs. And the big finale cranks it up a little more with the hero grabbing the villain and forcing as many teleports as possible, purposefully meshing the two with garbage, roaches and detritus until they're just a huge mutant blob. With roaches sticking out of them. Augh.
Another Koontz novel Midnight features (among other horrors) people who alter their own bodies to merge with their computers - and try to assimilate others into their networks. At one point one of them is shot in the head. His body dies, but immediately the computer screen starts printing out "Where is the rest of me? Nonononono!
Yet another two Koontz novels - Fear Nothing and Seize the Night - are on the premise that people can randomly start transforming into things, sometimes multiple things.
Ray Bradbury's "Fever Dream" is the story of a little boy who discovers that every cell in his body is slowly being replaced by... something, but nobody believes him because they think he's just delirious with sickness. The story ends with the boy having been completely replaced by the virus, with the parents none the wiser, and he's now a vector for the disease.
Another Ray Bradbury short story, "Skeleton", features a protagonist who develops a strange phobic disgust of his own bones— and whose bones may be objecting to it...
Although not necessarily horror, Ray Bradbury's "The Naming of Names" features humans who travel to Mars. They find that there once was a Martian civilization but it has long since died. Slowly but surely (but not necessarily painfully) the humans start becoming the Martians, physically, and mentally they start becoming the Martians as well. At the end a group of soldiers arrive from Earth and find that there is an abandoned human settlement but Martians are living in the nearby monastery. Its implied that the martians in question aren't aware that they were ever anything else, and one of the soldiers comments on how nice a place to live it looks...
The disease greyscale. It causes the flesh to calcify and crack, followed by insanity and eventually death.
The punishment for a whore caught infecting Tarly men is to have her privates washed in lye (potassium hydroxide). For the record, this is what lye does to a hand◊.
The way Vargo Hoat dies in A Feast for Crows.
Jean-Paul Sartre's existential novel Nausea gruesomely delves into imaginary body horror when the protagonist narrates his daydream of what might happen if reality suddenly began to defy people's expectations of it. A bloody lump of half-rotted meat dragging itself across the street, a child's cheek splitting open to reveal scattered eyes growing out of his face, and people waking up to find their tongues partly changed into wriggling centipedes are just the beginning of his imagined apocalypse. Even then, he realizes people would still find a way to categorize the world around them, coming up with new names and meanings for these horrors like "stone-eye, great three-cornered arm, toe-crutch, spider-jaw."
The natural creation of Whampyri in Brian Lumley's Necroscope series is pure body horror. A Whampyri is a result of a human being infected by a parasitic leach, which changes their body structure and heightens their lust and power over remorse. To create offspring, the vampire leeches of the Wamphyri create a single egg, which is then put into a potential host, called an egg son or egg daughter. The egg children usually lose consciousness during the transformation due to the pain of the egg merging with their bodies. Should a Whampyri die, the leech will try to abandon its host and search for a new one. An exception to the single egg laying Whampyri, are the "mothers" whom all Whamypri fear due to their ability to lay countless leech eggs (but not before being drained into a lifeless husk).
In "The Seed from the Sepulchre" by Clark Ashton Smith, there is a horrific man-to-plant transformation.
R. A. Lafferty's "Dream" is a nice Christmasy story where everyone suddenly starts to dream that they are hideous ogres, crawled over by bugs the whole time, whose digestive system consists of rats that run in and out of their mouths to bring food into their stomachs. But there's an uplifting ending: humanity realizes that their own fate is in their hands, and all they have to do to stop the hideous dreams is to decide, once and for all, that they want to wake up in the real world.
The mad dream disappeared as suddenly as it had appeared. The world came back to normal with an embarrassed laugh. It was all over. It had lasted from its inception six weeks.
The mad dream that vanished, however, is the dream that people weren't hideous ogres..
In the Cordwainer Smith story "A Planet Named Shayol", criminals are exiled to a prison planet inhabited by an alien parasite that keeps the prisoners alive but causes their bodies to grow extra parts - which the planet's single guard harvests when he visits the prisoners, and sends off-world to be used in organ transplants.
It's found in spades in the works of Sheri S. Tepper. In Shadow's End, in exchange for humans being permitted to live on the planet Dinadh, when a woman experiences her first pregnancy she is then gang-raped by a native race called the Kachis. Several Kachis grow in her womb, eating the human foetus for sustenance. When the woman goes into labour, if there isn't a special container to restrain the Kachis when they are born, they will proceed to attack the woman. In Sideshow a conjoined-twin brother/sister are attacked by the main villains and are converted in dinka-jins, artificially enhanced bodies (imagine a human body converted in mechanical parts that can detach themselves from the main core and move about independently). In Gibbon's Decline and Fall the main villain envisions a world where women exist in mindless suspended animation, the only part of their body utilised is the womb in order to create more men for his "perfect reality".
And then there's the professor with Voldemort's face protruding from the back of his head!
The Polyjuice Potion. (shudders)...
"Splinching" is what happens when a person attempts to apparate and accidentally leaves part of themselves behind. It's Played for Laughs at first...until Deathly Hallows, when Ron leaves behind a good-sized chunk of his arm, and nearly bleeds to death.
Nearly Headless Nick. Forty-five axe blows to the neck and his head still stays attached by a half-inch of skin!
In Harry Potter and the Chamber of Secrets, Harry breaks his arm. Lockhart tries to help, but instead of mending the bones, he removes them! Yeah, that's right, he removes all the bones from his arm, leaving Harry with little more than a tentacle with fingers at the end. And then Harry has to regrow the bones in his arms by taking Skele-Gro; Harry compared the healing process to having a million splinters lodged in his arm.
Harry Potter and the Prisoner of Azkaban has Professor Lupin's transformation into a werewolf. Also, the stories of his transformations as a child coupled with the bloodstains and destruction in the Shrieking Shack are pretty horror-inducing, especially as he has to go through a similar process every single month...
The Diamond Guardian's vicious and scary pets are biologically attached to him.
Almost happen to Lief if his companions didn't take off the mask given to him by the Masked One's chief.
In Lawrence Watt-Evans' Obsidian Chronicles, dragon venom that is ingested or absorbed into the bloodstream of humans grants them immortality while stealing many of their emotions and ability to reproduce. However, a final price is revealed in the first book when it turns out that the venom is in fact slowly transforming the human's heart. When the incubation is complete, the heart undergoes a sudden metamorphosis and an infant dragon tears its way out of the human host in a spray of gore.
In Julian May's Jack the Bodiless, the titular Jack starts to experience his terrible mutation at the age of two, as intractable cancers destroy his body. Genetic engineering helps at first, but in the end nothing can stop the transformation. Various scenes over the course of months point out the effects of the cancers, culminating with nothing left but his eyeless, skinless head. And then, in a moment of desperation, he completes what turns out to be an evolution, leaving behind his dying body to become an independent disembodied brain. It's implied that if the doctors had left well-enough alone, his transformation would have happen a lot quicker with less mess.
Not all. Other Father was turned into a grotesquely misshapen and blind creature, left to crawl around the basement, while Other Spink and Forcible's fates were similar, only they were trapped inside a cocoon. And let's not talk about what happens to them in the movie...
Parasite Eve Kiyomi's Mitochondria eventually control her, while her mind is still active. Yes, her MITOCHONDRIA. Her husband has the brilliant idea to keep her mitochondria alive (to be fair, he didn't know they were evil) and they end up possessing people, setting people on fire, and turning into what is described as Kiyomi, sans skin and with the ability to reform herself. And to top it all off, Eve rapes Toshiaki and impregnates a young girl with the "baby" and the young girl goes through nine months of Pregnancy in what can't be more then a half an hour (Shuddder) made worse by the novel taking time to explain concepts behind the science and Mitochondria really do have their own DNA.
1984 features this briefly when O'Brien criticizes Winston after several weeks of intense torture. To do this, he shows Winston his reflection in a mirror, revealing him to look barely alive and profoundly dehumanized. To demonstrate the extent of his physical decay, O'Brien rips out some of his hair and removes a tooth with uncomfortable ease.
Jonathan Shriek in Jeff VanderMeer's Shriek: An Afterword allows himself to be infected by a fungus to infiltrate the secret tunnels of the Graycaps. What starts out as bit of mould under his fingernails ends up in a state where he looks like an overweight middle-aged man as long as he concentrates, but if he allows the fungus to emerge, tendrils are involved. And occasionally he'll mention a bodypart like his ear, and comments that he actually lost it ages ago; it's just fungus mimicking the original form.
The following book, Finch, features Partials, kind of fungus-cyborgs that go through a similar process as Jonathan, but specifically allow a fungus eat out one of their eyes, and replace it with a spore-based camera.
JG Ballard, card carrying futurist that he is, has dabbled in this genre. David Cronenberg's Crash (see Film) is an adaption of one of his novels.
Dr.Krok from "Comrade Death", accidentally came into contact with a single, trivially small drop of the new poison he created. His body was bloated and deformed until he resembled a hippopotamus in the shape of a man, with bulging red eyes, his nose having been swallowed by his monstrous face, and no teeth. Had the drop been larger his body would have dissolved into liquid.
First, we have the Embrace of Pain. In fact, the Yuuzhan Vong in general are big on pain. It's not uncommon to see a Vong with dozens of piercings, some of them connected to other piercings by a too-short chain.
High-ranking Vong are fond of surgically removing parts of their own bodies and attaching parts from other creatures in their place. This, coupled with the ritual scarrings and piercings mentioned above, are particularly common among the priest and warrior castes (since all of this Body Horror is an act of worship towards the Vong gods, and those two castes are known for excessive displays of devotion); members of the shaper caste prefer to avoid this in favor of more subtle alterations to their internal organs.
Sith "alchemy" can border on this at times. Luke's clones in Dark Empire are a lot taller and more muscular (and dumber) than him. Luke's soul is actually severed from his body in the Jedi Academy Trilogy.
Not sure if it counts, but in "Tales of the Bounty Hunters", Dengar's backstory is that, after a crashed speeder, he was "fixed" by having the parts of his brain which showed emotions being slowly removed, leaving him a robot with no ability to care, no emotions beyond anger, hope, and (accidentally) loneliness. This was intentional: the people who "fixed" him wanted him to be made into an assassin with the hope that if he did well enough they would restore his other emotions.
Also in "Tales of the Bounty Hunters", most of Boba Fett's skin was slowly burnt away by acid. Admittedly they don't show this happening, just the aftereffect, but the implication is fairly horrendous.
Galaxy of Fear, being inspired by Goosebumps, has this now and then. The Planet Plague features The Virus which turns people into Blob Monsters in a rather disgusting way. Thirteen-year-old Tash is injected with some and throughout the book feels irritable and angry as a rash-covered lump on her arm grows and swells until it splits and starts oozing virus-laden slime that fights her movements.
The "Stuff" that fills The Gone-Away World typically turns into whatever someone's thinking of. If it gets on someone, it can turn them into whatever they're thinking of. Unfortunately, their internal anatomy is also matched to whatever they're imagining, so if they don't have detailed knowledge of the organs of whatever they're thinking of, odds are they're not going to survive the experience.
Infected and Contagious by Scott Sigler each involves bio-machines from outer space that use humans as hosts to grow...until they are mature enough to tear their way out.
During the first book in the Legacy of the Drow Series, Dinin Do'Urden adamantly refuses his sister Vierna's order for him to be part of a mission to capture their brother, the Defector from DecadenceDrizzt Do'Urden. Because he's fought the guy once, and he's scared of him. Vierna's response? She turns him into a drider, a bloated half-drow/half-spider hybrid.
To add to the horrific nature of normal Drow society, this transformation is not necessarily considered a punishment, and in some circumstances is seen as a great honor.
At the end of Descent Into The Depths Of The Earth, Escalla corners her sister after she's revealed to have been behind a plot to free an ancient and insane god from prison involving a ritual that requires mass human sacrifice, and incidentally have Escalla put to death. Escalla uses three medium-power spells to exact hideous vengeance: Flesh to Stone, Stone to Mud, and finally Dispel Magic. She's taken to prison in a BUCKET.
Just because I'm a little blonde Faerie I have to be nice?
The Stone Dance of the Chameleon runs on this trope. The Masters are completely obsessed with ritual mutilation. If common people see a Master unmasked, the least horrible punishment is being blinded. There's a caste of people who have one eye plucked out at birth. Likewise, pregnant women are sometimes administered a poison that makes them more likely to give birth to Conjoined Twins, one of which is always blinded at birth. Then there's a different people, the Marula, whose oracles commune with their god by having maggots burrow through their own flesh. Most grotesque of all, however, are the Wise, a political faction of the Masters. These people are stripped of all their senses except touch - eyes, nose and tongue and eardrums are cut out. They are also castrated and communicate with the outer world through a homunculus, a personal slave whose growth has been deliberately stunted. By pressing the homunculus' throat in a certain way, they can relay what they want to see through the homunculus, who will speak for them.
Haunted, Chuck Palahniuk's collection of short stories: Saint Gut-Free's incident with the pool filter, Comrade Snarky's mutilation, and Baroness Frostbite losing her lips to ... well, the obvious.
The Big Bad of Rivers of London takes possession of it's hosts, then turns their face into a lookalike of Mister Punch with the giant hooked nose and chin shattering the jaw and shredding the skin. Then when it is finished with its host, their face falls off. Oh, and they are still alive at the time it happens.
In the sequel Moon Over Soho the Black Magici...sorry, Ethically Challenged Magician keeps a severed head alive, conscious and enslaved for over four decades, plus has a sideline in creating real CatGirls by fusing people and with actual cats. There is worse, but Nightingale tells viewpoint character Peter Grant that he doesn't want to know and Grant decides to accept this since the clean up crew has to involve people who excavate war graves in Rwanda and Kosovo.
People who think Zenna Henderson was a gooey-sweet schoolmarm should read her non-The People stories. In "The Last Step", alien invaders are shooting people with dart guns. They seem to do no more than prick you, but wherever you are pricked swells to the size of an orange and hurts. It can be relieved by cutting it open, but this has to be done with extreme caution, because what's inside are a lot of tiny aliens who jump out and scramble away, their feet pricking your skin in the process. And then those pricked places....
The psychic children in Bitter Seeds have to carry big batteries around in order to fuel their powers. The batteries are plugged into their brains.
The warlocks opposing them on the other hand have to shed more and more blood every time they summon. First it's just a few drops. Then a fingertip. Then an arm. Then ...
The main protagonist from Max Barry's Machine Man. First he loses his right leg in an industrial accident, and builds a better prosthetic replacement. When he realizes he's done a better job than evolution, he returns for a matching set. That is truly only the beginning.
Dan Simmons' Hyperion Cantos series features the cruciforms, parasites that bond to a person's chest and create a network of tendrils that become coextensive with the host's nervous system. The cruciform will resurrect the host every time the host dies, but with each resurrection, the host's genetic code degrades slightly, to the point where the host becomes deformed and severely mentally disabled. They also confine the host to a relatively small geographic area by inflicting excruciating pain whenever the host tries to travel too far from the cave where the parasites are incubated.
The shared-universe series Wild Cards. An alien bioweapon is released over New York. 90% of humans are unaffected. 90% of the affected die. 10% of the survivors gain some kind of superpower and look normal. The other 90% of the survivors are pure nightmare fuel.
In Yann Martel's Self, after being raped, the trauma of the event causes Yann's body to revert back from female to male. As if the detail used wasn't creepy enough, Yann is pregnant, and the fetus now has to find somewhere else to exist inside Yann.
What theChaga does to terrestrial life in Ian McDonald's short story "Toward Kilimanjaro" and his novel Evolution's Shore. It's arguably beneficial. Usually.
German writer Walter Moers wears this trope thin. In The City of Dreaming Booksthe Shadow King is turned from a young aspiring human author into a creature made entirely of paper which combusts when it encounters light. We are also treated to the full process of how it happened - which included being boiled alive, dismembered and watching bits and pieces of yourself float about in tanks around you.
In Rumo & His Miraculous Adventures there's a whole army of "Copper Killers" - soldiers who died in battle and were then, with the help of alchemy, clockwork and science, reanimated as immortal hybrids of flesh and technology.
Clive Barker isn't entirely unfamiliar with this trope, especially in his Books of Blood collection. "The Body Politic" has a set of hands - followed by all other hands - attaining sentience and deciding they want "autonomy." "Jacqueline Ess - Her Will and Testament" features a housewife who survives a suicide attempt and gains the ability to warp flesh, using her ability to grant men a terminal and ecstatic sexual experience.
His Abarat series definitely qualifies, especially with the accompanying illustrations. The worst has to be the Fugit brothers, whose facial features are mounted on insect legs that they use to crawl around.
According to a scene in BIONICLE Adventures #6: Maze of Shadows, Makuta Teridax's abandoned lair is littered with his experiments, in the form of the strangest of dying or dead animal hybrids. And in Bionicle Legends #9: Shadows in the Sky, another Makuta, Icarax, experiences what it feels like to become an example, after his soft tissues and organs are squished by his armor, and then has to stay that way.
The Devil's Alphabet by Daryl Gregory. Most of a town's inhabitants are struck by a strange plague that mutates them rapidly. Aggros become about 9 feet tall with grayish skin,aggressive tendancies and no body hair. The protaganist's mother was one of many who didn't survive the transition, and she, like many others, experienced intense pain and ended up with bone jutting out of her limbs because the bone growth was so rapid that the muscles and skin couldn't keep up. The plague shifted and the next group were the sealskinned Betas, all female save for a single male baby born at the end. The third group, including the main character's father, became grotesquely obese Charlies. They don't smell too good and they develop large, rapid growing, fluid filled pustules that are drained by Charlie boys so they can get high off the liquid inside. The protagonist is horrified watching one of the blisters grow before his eyes.
Pure by Julianna Baggot. Imagine a nuclear sort of blast that fuses you to whatever you were touching when it happened. The protagonist had a doll's head fused to her hand. Her dad had a fan fused to his neck. Some people had animals or other people fused to them. Mothers were fused to children. A guy had a dog fused to his leg and another had birds stuck to his back. Sometimes even whole groups of people were fused together, known as 'groupies'. And some melded with the dust, hence their name 'Dusts'.
F. Paul Wilson is fond of this. One particularly horrifying example was what happened to Danny Gordon in Reprisal.
In Weavers of Saramyr, The Weavers' True Masks give their wearers great power while simultaneously taking everything from them, both mentally and physically. The longer a Weaver wears a Mask, the more grotesque and diseased he becomes, to the point where Vyrrch is basically melted and flabby on one side of his face, while the other is missing most of its skin, revealing parts of the skull underneath. The book's description is much, much more disgusting than this..
In The Changeover, 'Sorry' Carlisle claims he felt the protagonist Laura's skull shifting when she underwent the titular changeover.
The final book in The Edge Chronicles series, "The Immortals", introduces a condition known as 'Phraxtouch'. Miners mining Stormphrax (a rare material that can purify the filthiest water) end up inhaling tiny fragments of it, and over time it builds up and slowly purifies their blood into water, killing them. A tell-tale sign of someone who's been phraxtouched is a vapor that is constantly exhaled from the mouth.
The Gone series by Michael Grant has too many to list without it becoming a folder on its own, but here is just a few examples of the less disturbing metamorphoses:
Once Mary Terrafino and Francis emerge on the "Other side" they are mangled, have lost their mouths and eyes and are apparently "too disgusting" to describe in detail. The person who found Mary called animal control first, because he thought she was a dead bear!
In "Plague" both Dekka and Hunter have bugs hatch out of their bodies and feed on them, crippling them both. For Dekka, temporarily.
Also from Plague Brittney/Drake being cut in three by Brianna.
Brittney/Drake goes through a lot, starting right when they got to claw their way out of a grave.
E.Z getting eating alive from the inside out by mutated killer worms in Hunger.
In Fear, Cigar is left at Penny's mercy from sunrise to sunset, and she, among other brutalities, makes him claw his own eyes out.
James Patterson liked this in the Maximum Ride series, mostly in the lab scenes. One had a vaguely person-like...thing...that appeared to be one massive fungus.
In Robert Reed's Great Ship universe, the Remoras - a Human Subspecies - are constantly bombarded by interstellar radiation due to them living on the outer hull of the Great Ship, which causes rampant mutation. However, they deliberately cultivate their mutations through use of technology into forums they consider useful, or beautiful. One Remora character is described as having no eyes, but instead having glowing hairs poking out of where their eyes should be.
The whole idea of This Book Is Full of Spiders is an alien parasite that crawls into a person's body, takes the place of a body part, and mutates them, often in painful and illogical ways, into monsters. Also done to a lesser extent in the first book of the series, John Dies at the End.
Many examples in The Book of Lost Things abound but the Huntress' "creations" and the Crooked Man's body falling apart come to mind.
The Power of Five: Diego Salamanda forcibly mutilated as a baby. His head was put between two planks and forced to grow up. By adulthood, his head was nearly twice as long as the average man and his face is equally deformed.
Stephen Monaghan, hero of The Dollmaker, has his blood turned into something either like breast milk or semen, depending on how squicked you want to be.
David Tirado in Everyman slowly turns into a huge colony organism of the doppelganger's victims. Tons of juicy body horror ensues.
In the Ravenloft novel Death of a Darklord, a wizard tries to use magic to heal people, but one of her patients grows scales over his wounds, another has a miniature conjoined twin growing out of his neck, and another has a tentacle for an arm.
The Changing from The Maze Runner Trilogy. Dear God, The Changing. note To elaborate, it's a side effect of the [Anti-Griever] Serum that makes your veins green and pop out. The good news is that the sheer pain of the process knocks you out.