Disease Bleach

"They used to call me 'Red', but three years of malnutrition will do a number on your hair color."

—Ian Starshine, The Order of the Stick

It's a well-known fact that stress causes hair to turn gray—just ask your grandmother. Stress, however, comes in many forms, and some are more effective than others at turning hair white.

One of the most effective ways to turn a character's hair completely white would be the stress of prolonged illness. There may be some Truth in Television to this, but typically it's taken to extremes. In some cases, this trope is used to justify the white hair of a White-Haired Pretty Boy if he happens to also be an ill guy.

Note that this trope also applies to people whose hair has turned white due to prolonged stresses similar to diseases, such as being a prisoner of war. If a character's hair suddenly changes color forever due to supernatural causes, it's Locked Into Strangeness. If this is a direct (often temporary) effect of supernatural influence, it's Power Dyes Your Hair. Also has no relation (usually) to Brain Bleach.

Also note that in Real Life hair already grown can't turn white (unless you bleach it artificially). Grown hair consists of dead cells, which can't change their color. But don't expect fiction to care.

Examples of Disease Bleach include:

Anime & Manga

Jushiro Ukitake of Bleach has his hair turn white after a sick spell. Don't ask us how ghosts get sick, or change hair colour.
In Death Note, Soichiro Yagami's hair starts greying at an alarmingly fast rate from the moment he begins to work on the Kira case. Especially notorious in the first arc, where about a third of his hair turns grey in a couple of episodes, way too fast for it to be caused by simple aging. Until about episode 10, his hair is pitch black. Though he didn't start greying until after he had a stress-induced (rather than a Kira-induced) heart attack.
Toki from Fist of the North Star used to be black-haired before being irradiated, after which his hair turned white.
Half of Dr. Black Jack's hair started coming in white after a land mine pretty much blasted his head apart when he was a child.
Mushishi's Ginko got white hair from a magical disease which also took one eye and his mentor's life, just as will happen to him once his other eye vanishes.
Victorique from Gosick suffers this in the ending of the story. It was likely a combination of an already crappy childhood, added to the stress of being a criminal refugee and having to flee to the other end of the world alone in the middle of WWII.

Comics -- Books

The title character of XIII has a grey streak on his left temple where a bullet scratched him pre-amnesia.

Films -- Live-Action

This happens to Johnathan Harker in Bram Stokers Dracula as his hair turns grey when he returns to London and then white as he aids in fighting the count.
The villain of the Saw franchise for the first three films has white hair at a relatively young age (canonically in his mid-forties during Saw III) due to being terminally ill with cancer.
Will Graham in Manhunter has gray tinges in his hair due to his experience with Hannibal Lecktor.
In X Men, after being forced to absorb Magneto's power, Rogue acquired a permanent silver streak in her hair, to match the white streak Rogue sports in the comic books.
The vampiric disease in The Omega Man turns people's hair white instantly (as we see when Lisa gets infected). And curing it turns their hair back to a normal color instantly. Somewhere A Virologist is Crying.

Literature

In the most recent Dresden Files novel, we see that Lea has white streaks in her hair after prolonged imprisonment by the Winter Queen in order to "cure" her insanity/ambition.
In Alexandre Dumas' The Count of Monte Cristo, Danglars ends up a broken man with completely white hair following a prolonged period of psychological torture by the Count.
Percy Jackson and The Titan's Curse gives Annabeth and Percy cute little matching gray streaks from the stress of holding up the world. Yeah, my hair would go white then too.
- Though oddly enough, they're never brought up again afterward. Maybe they dyed them to blend in-?
In Dean Koontz's Dragon Tears, Harry Lyon's former partner Ricky Esteban had to retire from the police force on medical grounds after being gut-shot, subsequently losing a big chunk of his digestive system. His hair went white and he seems prematurely aged in other ways, not surprisingly.
In A Song of Ice and Fire, Lancel Lannister's near-mortal wound and long recovery are said to have changed his fine blonde hair to a brittle white mess.
- In A Dance With Dragons Theon's hair turns white as a result of the malnutrition and torture he suffers from Ramsay.
Roderick Usher in Edgar Allan Poe's story The Fall of the House of Usher, making this trope Older Than Radio.
Something like this happens to Erast Fandorin in the first book of his series, following the death of his new bride in a bombing, as he ends up with grayed temples.
In Bram Stoker's Dracula, Jonathan's hair begins to go grey (or possibly turns entirely white) after his imprisonment at the Count's castle. It's believed by some to be another way in which he grows to mirror Dracula himself, similar to his personality change and growing fixation on his weapons.
Felix Phoenix of Tanith Lee's Piratica used to have dark hair, until years in a workhouse as a child turned him into a White-Haired Pretty Boy.
In Les Misérables, during the trial of "Jean Valjean", as Monsieur Madeleine sits in the audience, his hair turns from graying to snow-white while he awaits the moment to reveal that—super awful spoiler here -- HE is Jean Valjean!
This happens to the Judge himself in the Judge Dee novel The Chinese Nail Murders. At the end of the novel, which has been filled with tragic events and a crisis which threatens his career, he is congratulated by an Imperial official who refers to him as an old man. Dee is about to take offense, being only in his 40s, but then sees his reflection and realizes he has gone completely gray.
Jem in the Infernal Devices series. Turns out it's because of a demonic halluginogenic drug originally used to torture him as a child. Same goes for his eyes.
The strain of the often lethal Trial of Grass left young Geralt with white hair and unnaturally pale skin.
After the near-death experience that he has in Pastwatch: The Redemption of Christopher Columbus by Orson Scott Card, Christopher Columbus's hair starts rapidly turning white.

Music

The Crash Test Dummies' song Mmm mmm mmm mmm has the first verse describe a kid who got into a car accident and ended up with white hair from the shock and injury.

Theater

In the Broadway version of Sweeney Todd: The Demon Barber of Fleet Street, Toby's hair turns completely white after he is locked in the evil basement by Mrs. Lovett and suffers a horrific Break the Cutie ordeal which consisted of learning what she and Sweeney got up to and then being stalked by Mrs. Lovett who intended to hand him over to Sweeney to be killed.
- The film version gives Todd himself a streak of white from the trauma of his incarceration.

Video Games

In Phoenix Wright: Ace Attorney, Godot's hair turned white after he fell into a coma after being poisoned. Then again, this may be the most realistic side effect of his coma.
- No, that would be the blindness. Now, the visor, that's just silly.
According to the official BlazBlue Setting Material Collection, the attack that got Ragna's sister kidnapped, burned down the orphanage his family lived in, and cost Ragna one of his arms is also responsible for turning his blond hair permanently white.
- Well, to be more specific, it wasn't until Rachel took it upon herself to bite him and essentially afflicting him with vampirism that caused his hair to turn white as a result.
In Warcraft 3, Arthas' hair goes white after his experiences in Northrend, though it's not entirely clear if it was the result of the magical influence of Frostmourne or simply stress, obsession and trauma combined with frigid weather.

Web Comics

In The Order of the Stick, Ian Starshine's red hair turn white after three years of malnutrition in the Empire of Blood's jails.

Real Life

While not from illness, Senator John McCain's hair turned white at the age of 31 when he was a POW.
Said to happen to U.S. Presidents (and by extension, probably other world leaders as well). Apparently, presidents age roughly two years for every year they're in office—with associated white hair. Bill Clinton, for instance, went into the Presidency with a head of mostly dark hair and came out of it eight years later with it mostly white.
- Mr. Barack Obama rapidly greyed, as well.
- Abraham Lincoln, anyone?
- George W. Bush looked much younger upon being sworn in as president in 2000 than when his term ended.
- Probably averted with Donald Trump, who was 70 when elected in 2016 and is known to dye his hair out of vanity.
Barbara Bush, wife of the first President Bush, infamously had white hair by the time she was 30. This is mostly credited to the death of her daughter at the age of 4 from leukemia.
Al Capone claimed his wife was getting gray hair in her 30s because she was so worried about the press and police "harassing" her husband.
Marie Antoinette may be the poster girl, if you take credit from the legend: the night before her execution, she was blonde. The next morning, every hair of her head was white. In her defense, she had been through a lot of bad stuff before that.
- She wore a wig.