Beneficial Disease

One of the characters has a disease that is useful in some way. Perhaps it protects him/her from another, more deadly disease. In other cases, the disease might be fatal but give the person extraordinary powers.

This trope is related to diseases that can be cured by either a medical treatment or the body's self-defense mechanism. The characters may elect to not cure or prolong the infection to gain the maximum benefit from the disease. For immunity/superpowers arising from disabilities of a more permanent nature, (disabilities caused by genetic mutations, accidents or general inborn traits), see Disability Immunity. For example, sickle-cell anemia that arises because of a genetic mutation resulting in lowered life expectancy, but also provides a degree of immunity against malaria, is Not an Example of this trope. It is an example of Disability Immunity, since sickle cell anemia is permanent. On the other hand, cowpox, which is a disease that provides immunity to a much deadlier disease called smallpox, is a good example of this trope.

Another potential case of this trope could perhaps be in Video Games or Tabletop Games, wherein the rules could state that they could only have one disease at a time, so having a lesser disease can be beneficial because it prevents you from being infected by worse diseases.

Also related to One Curse Limit.

Advertising

 * In a candy bar commercial, Bart Simpson is diagnosed with Butterfingeritis. Homer grumbles, "Why does Bart always get the good diseases?"

Anime and Manga

 * Baoh is aboht a man with a parasite which gives him super-powers but is still eventually fatal.
 * Matou Kariya from Fate/Zero is infected with worms that will ultimately kill him, but infuse him with enough magical power to summon and control Beserker.

Comics

 * No less than three of Empowered's teammates got their powers from alien venereal diseases.
 * Blake and Mortimer; a plot that hinges on  is foiled when one is revealed to have sickle-cell anemia, preventing the disease from developping.

Fanfic

 * There's the Harry Potter fanfic "Recnac Transfaerso", where having cancer causes Harry to develop on-off superpowered magic.

Film

 * In Phenomenon, John Travolta's character develops hyperintelligence and even psychokinesis because of what is eventually discovered to be a terminal brain tumor.
 * Resident Evil Apocalypse. Alice gains superhuman strength, speed and agility because the T-virus that infected her has bonded with her on a cellular level.

Literature

 * The science fiction novel The Skinner by Neal Asher is set on a Death World that has this gigantic leech whose bite carries a virus with interesting properties, rendering "victims" super strong and nearly immortal and indestructible. Almost all animals on-planet are infected by it, as are most humans who live there. The real downside is that the evolutionary "purpose" is so predators can have permanent prey, and even if you can't die, it doesn't mean you can't feel pain/suffer a Fate Worse Than Death.

Live Action TV

 * Possible Trope Codifier in the Red Dwarf episode "Quarantine", which features the crew discovering various positive viruses such as "Luck", "Joy", and "Sexual Magnetism". They make appearances for the rest of the series.
 * This trope has been used multiple times on House.
 * In Terra Nova, a flu infection provides the main character immunity from another infection that wipes the person's memory
 * In an episode of Stargate SG-1, the characters receive a number of armbands that bestow superpowers on the wearers. They work by infecting the wearer with a virus that causes the changes. Unfortunately this means that the armbands only work for as long as it takes the body to develop an immunity to the virus.
 * In "The Broca Divide," Daniel and the Dr. Fraser's allergies make them immune to the week's malady because of the antihistamines they take.
 * Stargate Atlantis has a downplayed example. Sheppard finds himself resistant to Lucius Lavin's mind-control pheromones, because he has a cold and can't smell anything.
 * Look Around You has a disease called "Cobbles", which causes the skin to take on the appearance of stone until the victim looks like a pile of rocks, but also grants the ability to fly. The scientist who discovered a cure for the disease, a sufferer himself, opted not to use it because he liked being able to fly so much.
 * An episode of Smallville featured a little boy with a brain tumor that gave him telepathy. It's revealed to be fatal in a later episode, however, and they are unable to reach an expert who could possible save him before it's too late.
 * One the abducted women in the Criminal Minds episode "The Uncanny Valley" was diabetic, which somehow allowed her to metabolize the paralytic drugs she was given at a faster than usual rate.

Tabletop Games

 * Several "evil" diseases in Dungeons & Dragons provide growing bonuses at the cost of penalties in other areas, in effect becoming a Deadly Upgrade. While people are quick to point out that prestige class cancer mage can adapt to diseases, hoarding bonuses while ignoring the penalties, and break this in two it doesn't even require anything more than lesser restoration (a common staple spell) to acquire infinite strength at no penalty.
 * In Warhammer Fantasy Battle, this is Nurgle's hat. Since he is a Plaguemaster god, his servants become ravaged with all sorts of plagues but the effects don't kill them. They look utterly disgusting but not a bit weaker for it; they are actually harder to kill because they don't need to worry about things like infected wounds. Also they Feel No Pain, and the diseases they spread can still be lethal to non-believers.

Video Games

 * From Generation II onward, the main Pokémon video games have had the Pokérus virus. If you're very, very lucky, a wild Pokémon you fight might just spread Pokérus to one of your Pokémon. With this condition, that Pokémon will gain twice as many effort points (effort values are a complex hidden stat-growth mechanic, look it up) when an enemy mon is defeated. Basically, it will save you time when trying to fine-tune your Pokémon's stats. It can be spread to any Pokemon in the trainer's party who hasn't been infected before. Pokérus does, however, "cure" after so many hours of play, and though the effect never goes away, it can't be spread anymore. A way to avoid this is to keep a Pokémon with the virus in your PC, where Pokérus will stay active indefinitely.
 * Related, though not precisely this trope (since they aren't exactly diseases per se): Pokémon can only have one of six non-volatile status ailments: Burn, Freeze, Paralysis, Poison, bad Poison (most prominently from the move Toxic), and Sleep. A Pokémon may only have one of the six at a time, and with the exception of turning Poison into bad Poison, it's impossible for an enemy Pokémon to inflict a different one of the six on a target. Thus there are strategies like having a Pokémon hold a Flame Orb (which gives it the Burn status at the start of battle) to prevent other, more limiting status ailments from being applied.
 * Appears several times in the Trauma Center series:
 * In Under The Knife 2, all strains of Neo-GUILT grant the host benefits such as making them more intelligent, faster, or stopping the aging process. Well, until they get activated, that is...
 * In New Blood, while Master Vakushti's Cardia infection altered his personality, it also kept his life-threatening spinal necrosis in check. In fact, he promptly dies soon after Cardia is defeated.
 * In Trauma Team, While the parasite formed by the two becomes a deadly threat, it also makes the latter, until then incurable, take a shape that allows CR-S01 to eliminate it with ease.
 * The Elder Scrolls has vampirism, in which the longer it goes untreated, the victim gains more vampiric characteristics.
 * Also in The Elder Scrolls, particularly Morrowind, the Corpus disease grants the victims immunity to all other diseases and even prevents them from aging. Too bad it also comes with a big serving of Body Horror and a bad case of crazy, and is completely incurable unless you are the Nerevarine.
 * Possible that the Nerevarine was actually just the first person to be "cured".
 * He technically wasn't even cured, he just had the bad sides removed.
 * Skyrim adds Lycanthropy to the mix. You can turn instantly into a giant furry killing machine, you are entirely immune to all diseases (even vampirism) and the only downside is losing the ability to gain sleep buffs. Oh, and Hircine gains your soul when you die so that your afterlife will be an eternal hunt for game as part of his pack, though some people considers that yet another form of Cursed with Awesome.
 * In Dawn of War 2: Retribution, the healing of chaos units is done through the powers of Nurgle, by means of supernatural disease—Nurgle's Rot. The infected units get back to the fight as their senses get numbed to the pain and their wounds get sealed by cancerous growths.
 * There are a couple of these in Space Station 13, such as Owns Syndrome, which heals, confers stun recovery, and gives free sunglasses.

Web Original

 * The SCP Foundation notes that people with diabetes are somehow immune to the Spontaneous Human Combustion Virus.

Western Animation
"Mr. Burns: So... I'm indestructible Doctor: Oh goodness no, the slightest breeze could... Mr. Burns: Indestructible."
 * Futurama, "Parasites Lost": Eating a bad sandwich gives Fry worms that rebuild his body, making him stronger and smarter.
 * Mister Burns is apparently alive only because he's got so many diseases that they're all blocking each other out from outright killing him.

Real Life

 * Cowpox, a relatively harmless disease in humans, conveys immunity to the much more serious smallpox. This fact led to the discovery of the science of vaccination.
 * Malaria helps you deal with syphilis (High fevers can get rid of syphilis, and Malaria was deemed the optimal way of inducing the fevers for this purpose, since the fevers were both long and high, and then the Malaria itself could be treated with quinine). Someone even got a Nobel Prize for the discovery. Of course nobody cares now because of the discovery of penicillin.
 * Syphilis in turn can, but usually doesn't, turn out positive as in end state it alters your brain chemistry. It usually slowly kills you, but in some people it made them more passionate, generally better-mooded and enjoy emotions more (including of course sex, ironically enough).
 * Intestinal parasites such as tapeworms have been shown to reduce vulnerability of the host to airborne allergens such as pollen.