->'''Doctor:''' Mr. Burns, I'm afraid you are the sickest man in the United States. You have everything.\\
'''Mr. Burns:''' You mean I have pneumonia?\\
'''Doctor:''' Yes.\\
'''Mr. Burns:''' Juvenile diabetes? Hysterical [[MPreg pregnancy]]?\\
'''Doctor:''' Uh, a little bit, yes. You also have several diseases that have just been discovered - in you.\\
'''Mr. Burns:''' I see. You sure you haven't just made thousands of mistakes?\\
'''Doctor:''' Uh, no, no, I'm afraid not.\\
'''Mr. Burns:''' This sounds like bad news.\\
'''Doctor:''' Well, you'd think so, but all of your diseases are in perfect balance.
-->-- ''WesternAnimation/TheSimpsons'', Season 11 Episode 12: "[[Recap/TheSimpsonsS11E12TheMansionFamily The Mansion Family]]"

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 DisabilityImmunity. 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 NotAnExample 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 VideoGames or TabletopGames, 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 OneCurseLimit. Compare CurseThatCures, when a sick or injured character seeks out a {{Curse}} because it will cure them as a side effect. When something ''spreads'' like a disease but doesn't have harmful effects, see TheSymbiote.
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!!Examples:

[[foldercontrol]]

[[folder:Advertising]]
* In a candy bar commercial, [[WesternAnimation/TheSimpsons Bart Simpson]] is diagnosed with Butterfingeritis. Homer grumbles, "Why does Bart always get the ''good'' diseases?"
[[/folder]]

[[folder:Anime and Manga]]
* ''Manga/{{Baoh}}'' is about a man with a parasite which gives him super-powers but is still eventually fatal.
* Matou Kariya from ''FateZero'' is infected with worms that will ultimately kill him, but infuse him with enough magical power to summon and control Beserker.
* A ''TokyoMewMew'' episode featured a Chimera Anima with a smelly fart attack. Mew Pudding just happened to have a cold that day, so...
* In the 25th episode of ''Mushishi'', "Eye of Fortune, Eye of Misfortune", a woman was blind until a mushi entered her eye and allowed her to see again. Of course, it also eventually allowed her to see through objects and then into the future, before [[spoiler: ''crawling out of her head and taking her eyes with it'']]
* The Eclipse Virus in ''Manga/MagicalRecordLyricalNanohaForce'' kills its victims horribly unless they drain life from others. It also grants them AntiMagic and other powers to make the life draining easier.
[[/folder]]

[[folder:Comics]]
* No less than ''three'' of Comicbook/{{Empowered}}'s teammates got their powers from ''alien venereal diseases.''
[[/folder]]

[[folder:Fanfic]]
* There's the ''Literature/HarryPotter'' fanfic "[[http://www.fanfiction.net/s/1399984/1/ Recnac Transfaerso]]", where having cancer causes Harry to develop on-off superpowered magic.
* In ''FanFic/TimeWillTell'', Jorryn gets such a disease. The symptoms worsen until Jorryn {{faint|ing}}s in the USA. She wakes in the Shire (from ''Literature/TheLordOfTheRings''). This is a benefit because she can meet her favourite hobbits.
* The live vaccine against [[SyntheticPlague synthetic]] HatePlague called Madsen's Hollow in ProjectTatterdemalion is arguably this. It gives you fuzzy CombatTentacles, retractable, razor-sharp claws; [[YouGottaHaveBlueHair technicolor]] [[AlienHair tendrils]] in place of hair, PsychicPowers, the ability to sense and generate {{EMP}}s and use them to [[InnateNightVision navigate]] and communicate not entirely unlike [[{{Telepathy}} telepaths]] (sometimes to the point of MentalFusion if the tendrils get close enough to interweave); FlashStep, [[GoodThingYouCanHeal an amazing healing factor]], predatory instincts, and [[TrueCompanions instinctively bond with other shinigami]].
[[/folder]]

[[folder:Film]]
* In ''Film/{{Phenomenon}}'', Creator/JohnTravolta's character develops hyperintelligence and even psychokinesis because of what is eventually discovered to be a terminal brain tumor.
* ''Film/ResidentEvilApocalypse''. Alice gains superhuman strength, speed and agility because the T-virus that infected her has bonded with her on a cellular level.
* ''Film/{{Doom}}''. The Martian virus either turns you into a horrible monster, or gives you superstrength, limited bursts of speed and insane hand-eye coordination. The monsters know who will get which effects before trying to infect them. [[TheHero Reaper]] didn't know, but he sure benefited from it.
* In ''Film/{{Daybreakers}}'', vampirism is a disease which apparently started with bats. It gives the classic vampire weakness to sunlight, but cures all sicknesses, including cancer, and stops the aging process, as long as the recipient continues to have some human blood every day.
[[/folder]]

[[folder:Literature]]
* The science fiction novel ''The Skinner'' by Neal Asher is set on a DeathWorld 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 FateWorseThanDeath.
* At one point in ''Literature/TheDresdenFiles'' Harry is captured by vampires while dying of poison, with the result that the vampires can't drink his blood without becoming poisoned.
* This is pretty much the entire plot of ''Peeps'' by Scott Westerfeld.
* In the new ''Foundation Trilogy'', there is a disease called brain fever. Experienced by virtually all highly intelligent people, it seems to blunt intelligence but has a key beneficial effect for Hari Seldon [[spoiler:it makes him immune to R. Daneel's powerful Giskardian telepathic powers]].
[[/folder]]

[[folder:Live Action TV]]
* Possible TropeCodifier in the ''Series/RedDwarf'' episode "Quarantine", which features the crew discovering various positive viruses such as "Luck", "Joy", and "Sexual Magnetism". ([[CasanovaWannabe Cat]] comments: "Sexual magnetism's a virus? Then get me to a hospital, I'm a terminal case!") The are used in a later episode to help our heroes escape (and Rimmer uses Sexual Magnetism for...well, it's obvious.)
%%* This trope has been used multiple times on ''Series/{{House}}''.
* In ''Series/TerraNova'', a flu infection provides the main character immunity from another infection that wipes the person's memory
* In an episode of ''Series/StargateSG1'', 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.
* ''Series/StargateAtlantis'' has a [[DownplayedTrope downplayed]] example. Sheppard finds himself resistant to Lucius Lavin's mind-control pheromones, because he has a cold and can't smell anything.
* ''Series/LookAroundYou'' 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 ''Series/{{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 ''Series/CriminalMinds'' episode "The Uncanny Valley" was diabetic, which somehow allowed her to metabolize the paralytic drugs she was given at a faster than usual rate.
* In the ''Series/BuffyTheVampireSlayer'' season 2 episode "Killed by Death", Buffy's flu [[CaptainObvious makes her sick]], but also allows her to see the [[MonsterOfTheWeek demon of the week]], which had been killing sick children in the hospital [[FridgeHorror for years.]].
[[/folder]]

[[folder:Tabletop Games]]
* Several "evil" diseases in ''TabletopGame/DungeonsAndDragons'' provide growing bonuses at the cost of penalties in other areas, in effect becoming a DeadlyUpgrade. The fans were quick to notice that the prestige class cancer mage ''from the very same book'' can adapt to diseases, hoarding bonuses while ignoring the penalties...
* In ''TabletopGame/{{Warhammer}}'', 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 FeelNoPain, and the diseases they spread can (and generally will) still be lethal to non-believers.
* While most werecreatures of TabletopGame/WerewolfTheApocalypse are born as such, Ratkin are an exception: They're ritually infected with a disease called the Birthing Plague. Only those related to Ratkin have a chance of surviving it and, after several weeks of hallucination and homicidal rage, becoming wererats themselves. The plague still runs in their blood and renders them immune to any other disease.
[[/folder]]

[[folder:Video Games]]
* From Generation II onward, the main ''Franchise/{{Pokemon}}'' 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): Franchise/{{Pokemon}} can only have one of six [[StandardStatusEffects 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.[[note]]The Pokémon suffering from an ailment can use Rest, which will replace its existing ailment with Sleep.[[/note]] 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. Some effects, such as Poison Heal also make these status beneficial on top of granting immunities to the others.
* Appears several times in the ''VideoGame/TraumaCenter'' 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, [[DeadlyUpgrade 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'', [[spoiler:Naomi Kimishima, already weary from the GUILT she contracted during the events of ''Second Opinion'', gets infected with Rosalia.]] 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.
* ''Franchise/TheElderScrolls'' has vampirism, in which the longer it goes untreated, the victim gains more vampiric characteristics. But it also renders you immune to all diseases, which is a plus.
** Also in ''The Elder Scrolls'', particularly ''[[VideoGame/TheElderScrollsIIIMorrowind 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 BodyHorror and a bad case of crazy, and is completely incurable. Even [[TheChosenOne The Nerevarine]] wasn't technically cured, he just had the bad parts of it removed.
** ''[[VideoGame/TheElderScrollsVSkyrim 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 CursedWithAwesome. This might not even hold true for the Dragonborn, whose soul has, shall we say, some interesting strings attached.
* In ''DawnOfWar 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 ''VideoGame/SpaceStation13'', such as Owns Syndrome, which heals, confers stun recovery, and gives free sunglasses.
* In ''VideoGame/LionheartLegacyOfTheCrusader'', the Disease Ward perk grants your character passive healing when sick.
* The protagonist of ''VideoGame/DeepFear'' is immune to TheVirus because he's got [[spoiler: a cold]].
* This is part of the Quarians' backstory in ''Franchise/MassEffect''. As their home planet was full of beneficial diseases, their immune system grew to be very welcoming to foreign organisms. After their Geth rebelled and drove them from the homeworld, this became a fatal weakness, so they were forced to live in personal containment suits and keep their spaceships like a clean room.
[[/folder]]

[[folder:Web Original]]
* The Wiki/SCPFoundation notes that people with diabetes are somehow immune to the SpontaneousHumanCombustion [[http://www.scp-wiki.net/scp-081 Virus]].
** The SCP foundation also has captured a virus that causes you to regurgitate various objects depending on the situation. It can be very useful if you, say, lock your keys in your car and need another. But if a situation calls for something like a glass lightbulb, or a knife...
[[/folder]]

[[folder:Western Animation]]
* ''WesternAnimation/{{Futurama}}'', "Parasites Lost": Eating a bad sandwich gives Fry worms that rebuild his body, making him stronger, smarter and irresistible to the woman of his dreams.
* ''WesternAnimation/TheSimpsons'': 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.
-->'''Mr. Burns:''' So... I'm indestructible.
-->'''Doctor:''' Oh goodness ''no'', the slightest breeze could...
-->'''Mr. Burns:''' (walking off tenting his fingers) ''Indestructible.''
[[/folder]]

[[folder:Real Life]]
* Live virus vaccines operate on this principle, infecting you with a weaker version of the disease in order to build up your immunity to the full version of it.
* 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.
** You can still catch smallpox, even when vaccinated. The likelihood of it is significantly reduced, and your chances of surviving the disease are significantly increased. Also, the vaccination is only good for a decade at most. WHO doctors had to get the vaccination every five years at a ''minimum'' to maintain immunity until the disease was effectively eradicated.
* Malaria helps you deal with syphilis, (by causing such dangerously hot fevers that the syphilis bacteria can't survive) someone even got a Nobel Prize for finding out. Of course nobody cares now because of the discovery of penicillin.
** Speaking of malaria, those who carry the [[http://en.wikipedia.org/wiki/Sickle-cell_disease#Genetics Sickle cell trait]], they're immune to malaria.
* 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 sex - which is likely why it does it in the first place, because it encourages you to spread it more).
* Intestinal parasites such as tapeworms have been shown to reduce vulnerability of the host to airborne allergens such as pollen. They're also useful for [[http://www.foxnews.com/health/2010/10/04/parasitic-worms-cure-autoimmune-diseases/ treating Crohn's disease]].
* At least one allergy sufferer intentionally infected himself with hookworms in order to prevent allergic reactions by messing with his immune system. On the downside, he has hookworms.
* AIDS and other immunosuppressive diseases are very nasty, but they do have the minor upside of reducing allergic reactions due to the fact that your ruined immune system can no longer overreact to stimuli. There's even one girl who appears to have been cured of Leukemia by way of AIDS (it's an auto-immune disease, and it attacked the cancer cells).
** Some scientists are attempting to work out a way to [[http://www.sciencedaily.com/releases/2012/08/120828073304.htm cure cancer with AIDS on purpose]], which in medical terms is somewhat like summoning Cthulhu to fight Godzilla.
* Malaria parasites can't grow in sickle-cell affected blood cells. In heterozygous humans, it has few drawbacks, but in homozygous humans, it is itself a dangerous medical condition.
* There's a type of amoeba which, when it infests a human mouth, turns the teeth gray. This harmless cosmetic effect has its up side: the amoeba is a ravenous predator of bacteria which cause cavities.
* There is a kind of virus that infects tulips to give it color stripes.
* There's actually a condition called [[http://en.wikipedia.org/wiki/Heterozygote_advantage Heterozygote Advantage]], when someone may be a carrier of a recessive gene responsible for some life-threatening disorders, but they're immune to other diseases. For example, Chase, the daughter of [[Music/{{TLC}} Tionne Watkins]], inherited the genes responsible for sickle-cell disease from Watkins but is has a higher resistance to malaria. Similar to CelineDion, whose [[DownerEnding sister died of Cystic Fibrosis]], but carriers have higher resistance to cholera and tuberculosis.
[[/folder]]
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