Cure for Cancer

Marcus: Curing cancer, Mr. Wyndam-Pryce?
Wesley: Wouldn't be cost-effective. I'm sure we make a lot from cancer.

Marcus: Yes, the patent holder is a client.

—Angel, "Time Bomb"

A specific type of MacGuffin, the Cure for Cancer is something like a modern-day panacea. It is the ultimate medical achievement that everyone is looking for. Some people will want to sell it, some will want to spread it for free, and some will want to destroy it.

For some reason (half Fantastic Aesop and half Status Quo Is God), the Cure for Cancer often has some horrific side effect—it causes zombies, is made from people, or what have you. Often combined with Withholding the Cure.

Note that the reason we don't have this in Real Life (and the reason it is so sought-after) is because "cancer" is an extremely general term for any number of diseases. Some of them have actually been cured, especially on individual cases, but no miracle cure is forthcoming.

Minor variations include cures for other terminal, incurable diseases, such as Parkinson's and AIDS. As these are becoming more treatable, however, miracle cures for them are showing up less in fiction.

Examples of Cure for Cancer include:

Comic Books

During the "Dark Reign" storyline, Norman Osborne invented a cure for cancer... and immediately weaponized it in order to try to kill Deadpool - who had major blackmail material on him.
Wakanda in the Black Panther has had a cure for cancer for centuries, but they refuse to share it with the rest of the world because nowhere else deserves it.
- Note that this is part of a particular writer's run that may not be canonical as it contradicts many already established Marvel Universe facts.
There's a Superman What If story in which Lex Luthor apparently goes straight and starts using his brain for good, and if I recall correctly he finds a cure for cancer.
In Squadron Supreme, Tom Thumb travels to the future to steal the Scarlet Centurion's Panacea Potion (which can supposedly cure anything) to cure his cancer. However, Thumb discovered that the Potion consisted of no more than penicillin and a few complex vitamins; it worked with the people of the Centurion's time since over the many centuries the human species' immune systems had been improved through eugenics, but it was ineffective with people of the Twentieth Century.
In the Marvel Comics "Death of Captain Marvel" plot arc, all the genius scientist superheroes work together to find a cure for the dying captain's cancer. Which does bring up the question of why they didn't do that years before instead of spending their time beating up bank robbers? More annoyingly, because Reed Richards Is Useless, the cures they did find, but wouldn't work on Mar-Vell because of his powers, are never released to the general public (as seen by the Marvel writers never mentioning them again.)
- A recent storyline involved the "cancerverse"- an Alternate Universe where Captain Marvel's life was saved...by killing Death. It turned ALL life in the universe into immortal, cancerous beings. The cure was worse than the disease indeed...
In Marvel Comics, Venom feeds off of Eddie Brock's cancer when they're joined, keeping it at bay.
The plot of Doctor Strange: The Oath revolves around "Otkid's Elixir," a magic potion which Doc hopes will cure his manservant Wong's brain tumor. Naturally a corrupt pharmaceuticals company bent on Withholding the Cure interferes with him every step of the way.

Film

Medicine Man. Dr. Robert Campbell (Sean Connery) discovers a cure for cancer and then loses it.
In the Will Smith version of I Am Legend, a cure for cancer had the itty-bitty side effect of causing a ~~Zombie~~ Vampire Apocalypse.
The T-Virus in the Resident Evil Films was created to fix nerve damage. The end result of the virus was not what they'd envisioned.
The scientists in Deep Blue Sea were growing giant super-smart sharks to harvest chemicals from the sharks brains that could be used to cure Alzheimer. Unfortunately the process involved GROWING GIANT SUPER SMART SHARKS!
And in Rise of the Planet of the Apes they were growing super-smart apes as test subjects to find a cure for Alzheimer's, only to find that their latest drug 1) kills humans and 2) MAKES APES EVEN MORE SUPER SMART!

Literature

Succession mentions at one point that every time the main gun is fired, the crew develop "the simplest and most easily cured cancers" as a side effect.
In the novel The Child Garden by Geoff Ryman, cancer is eradicated—and it's discovered, too late, that it was the downside of an important part of the metabolism, which has also been eradicated in the process, drastically reducing human lifespan.
In the satirical book "Looking Backwards at the 80s (written in 1979) it's discovered that clubbing baby harp seals to death causes their brains to release a chemical that cures cancer.
'Cancer cure=zombies' also appears in the Newsflesh series.
In Octavia Butler's Parable of the Sower, a cure was made for Alzheimer's. Unfortunately, people without the disease started abusing it because of the enhanced mind capabilities it gave them, and this led to a generation of children with a disease called hyperempathy syndrome, in which they feel the perceived emotions of others.
In Never Let Me Go by Kazuo Ishiguro, the main characters are clones created to be used as unlimited organ donors for the cure for cancer.
In the Neil Gaiman short story Changes, a cure for cancer known as "rebooting" is developed, which, while it cures any case of cancer overnight, also has the minor side effect of switching the patient's biological sex.
One of Fred Saberhagen's Berserker stories had a damaged Berserker agree to negotiations with a human colony it couldn't be sure of killing in a straight fight. It took the opportunity to infect the human emissary with a bioweapon. But the weapon was tailored to this man's flesh, the only sample of human biochemistry the Berserker had, and ... "I think it's killing off my cancer."

Live Action TV

An episode of The Twilight Zone had an alien come to Earth, landing near a small Mexican town. The Mexicans distrusted him/it, and eventually killed it, but not before it tried to give them a gift in a book, which they burned. A nearby white scientist who had seen this happen but was held back from doing anything grabs the book and puts it out. He reads the inscription out loud.

"To the people of Earth. As a gesture of our goodwill here is the formula for curing all forms of cancer." The rest is burned.

In Stargate SG-1, the Goa'uld symbiont can cure cancer, among other diseases. Jacob Carter became a Tok'ra host for this reason.
The (initial) plot of Crusade revolved around a search for a cure for an alien plague.
In one episode of Seven Days, the cure for cancer mutates into a plague that wipes out all life on earth.

Memetic Mutation

Chuck Norris's tears can cure cancer. Too bad he has never cried. Ever.

Newspaper Comics

In the 4/17/11 installment of Curtis, a cure for cancer is found in the Film Within A Strip "The Clam." Too bad the scientist who discovers it turns into a giant clam before he can tell anyone.

Video Games

In some installments of the Civilization series, "Cure for Cancer" is a "wonder of the world" that a civilization is able to build.

Webcomics

In The Adventures of Dr. McNinja, Dracula discovered the cure for cancer and hid it on Mars.

Dracula: It's really funny, when you figure it out it's going to seem so obvious. But I don't want to give it away. It'll be really funny.

Keychain of Creation (an Exalted webcomic) has two demigods traveling Creation undercover.

Marena: That means no curing cancer willy-nilly.
Misho: Oh, but it's so easy once you know how to do it!
Marena: It's still not period, mister.

Western Animation

Near the beginning of the animated "Superman: Doomsday" film Superman is shown hanging out in the Fortress of Solitude trying to find a cure for cancer. Unfortunately he can never quite get it, and he wonders aloud how he can build a robot that can see the future but fail at curing a regular human disease.
A Robot Chicken sketch with the Popeye gang had a guardian angel show Wimpy a world where he never lived ala It's a Wonderful Life - except it was a virtual paradise where a cure for cancer had been found... by Alice the Goon!

Real Life

Cannabinoids. No, really.
Antimatter. No, really.