->'''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?\\
'''Doctor''': Yes.\\
'''Mr. Burns''': Hysterical pregnancy?\\
'''Doctor''': Uh, a little bit, yes. You also have several diseases that have just been discovered -- in you.\\
'''Mr. Burns''': I see. You're sure you just haven't made thousands of mistakes?\\
'''Doctor''': Uh, no, no, I'm afraid not.\\
'''Mr. Burns''': Well, this sounds like bad news.\\
'''Doctor''': Well, you'd think so, but all of your diseases are in perfect balance.
-->-- ''WesternAnimation/TheSimpsons'', [[https://www.youtube.com/watch?v=aI0euMFAWF8 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 them 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 cause 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. Contrast HarmfulHealing.


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

[[folder:Anime & Manga]]
* ''Manga/{{Baoh}}'' is about a man with a parasite which gives him super-powers but is still eventually fatal.
* Matou Kariya from ''LightNovel/FateZero'' is infected with worms that will ultimately kill him, but infuse him with enough magical power to summon and control Beserker.
* A ''Manga/TokyoMewMew'' episode featured a Chimera Anima with [[{{Fartillery}}a smelly fart attack.]] Mew Pudding just happened to have a cold that day, so...
* In the 25th episode of ''Manga/{{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.

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

[[folder:Fan Works]]
* 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 ''Fanfic/ProjectTatterdemalion'' is arguably this. It gives you fuzzy CombatTentacles, {{retractable|Appendages}} [[AbsurdlySharpClaws 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]].
** Though it's partially {{Justified}} in that Madsen's Hollow gives its victims almost all of those things as well, in addition to driving them insane.
* In the ''Literature/HarryPotter'' fanfic ''[[https://www.fanfiction.net/s/2163835/1/The-Refiners-Fire The Refiner's Fire]]'', Harry suffers the title illness, which nearly kills him as the magic in him gets "refined", but he survives and becomes a whole lot more powerful than the normal wizard. It is an illness so brutal that only two people before (Merlin and Dumbledore) have survived it, though, so it is less "beneficial" and more "reward for surviving what would normally kill you".

* 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. Failing to do so will advance the mutation so that vampires become mindless feral predators.
* According to the {{backstory}} of ''Film/{{Underworld}}'' is that the original immortal Alexander Corvinus became one after a deadly plague that wiped out pretty much everyone else in the area mutated in his body, giving him eternal life. Later, he had three sons, two of them inherited the active form, while the third was a normal human but with a latent form of the virus. The first two sons, twins, were bitten by a wolf and a bat, further mutating the disease into [[OurWerewolvesAreDifferent lycanthropy]] and [[OurVampiresAreDifferent vampirism]], respectively. Both species have superhuman strength and speed, as well as eternal life and perfect health. The only real disadvantages are weaknesses to silver (Lycans) and UV rays (vampires), although the original Lycans couldn't control themselves, transforming permanently in their lycan form and going on mindless violent rampages.

* The science fiction novel ''[[Literature/ThePolity 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 ''Literature/{{Peeps}}'' by Scott Westerfeld. Carriers of the parasite enjoy super strength, enhanced senses, and extended lifespan. Unfortunately most people who get infected become feral with a [[OurVampiresAreDifferent craving for blood]] and aversion to things they loved before their infection. And the parasite is very contagious, kissing is enough to infect someone.
* In the new ''Literature/{{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]].
* Lycanthropy in the ''Literature/AnitaBlake'' novels is depicted as a blood-borne disease that kills all other diseases and foreign substances (like toxins) in the carriers. You turn furry once a month, but can't ever get sick with anything else again. It's enough of a disease that there's even a vaccine, though a bad batch can actually turn you into a lycanthrope.
* In ''Literature/NakedLunch'', [[MadScientist Dr. Benway]] discusses curing several terminal catatonics by getting them addicted to heroin.
* In the ''Literature/{{Newsflesh}}'' world, both Marburg Amberlee (a cure for cancer) and "Kellis flu" (a cure for the common cold) were ''meant'' to be this trope. Unfortunately for that world, no one knew that when those two engineered viruses met, the result would be a ZombieApocalypse. A cancer- and cold-free ZombieApocalypse, granted.
* In ''Literature/TheFireman'' a deadly spore causes SpontaneousHumanCombustion. It is possible however to control the spore which in turn gives the host the power to create and control fire.
* A Cory Doctorow short story, "0wnz0red", features co-protagonist Liam, who's been implanted with a retrovirus linked to a microcontroller, essentially allowing bodyhacking. What does he do with it?
-->"First thing I did was reverse-engineer the interface bug. I wanted a safe virus I could grow arbitrary payloads for in my body. I embedded the antiviral hardening agent in the vector. Its a sexually transmissible wellness, dude. Ive been barebacking my way through the skankiest crack-hoes in the Tenderloin, playing Patient Zero, infecting everyone with the Cure.
--> Murray sat up and his head swam. You did what?
--> I cured AIDS. Its going around, its catching, you might already be a winner.

[[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!") They are used in a later episode to help our heroes escape (and Rimmer uses Sexual Magnetism for... well, it's obvious.)
* ''Series/{{House}}'' has used one disease to cure another on more than one occasion. Such as when the teenage faith healer whose touch seemed to cure cancer turned out to have herpes, which attacked tumors.
* In ''Series/TerraNova'', a flu infection provides the main character Jim immunity from another infection that causes amnesia and mental regression to an earlier period in the victim's life. Jim's wife, believing she's still in medical school, is still able to synthesize a vaccine when she realizes why he's immune.
* 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 Dr. Fraser's allergies make them immune to the week's malady because of the antihistamines they take,[[spoiler: conveniently preventing the only two named characters with the skills and knowledge to synthesize a cure from being afflicted by the evolutionary regression that turns everyone else into cavemen.]]
* ''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 possibly 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.
** [[TruthInTelevision The show did state that there was a significant chance of the drugs being absorbed into her system faster, killing her in less than a day, however.]]
* In the ''Series/BuffyTheVampireSlayer'' season 2 episode "[[Recap/BuffyTheVampireSlayerS2E18KilledByDeath Killed by Death]]", Buffy's flu 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: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...
** For that matter, Lycanthropy, at least in 3rd. Getting bitten by a "natural" lycanthrope causes you to turn into a slightly weaker (lower damage reduction and can't infect others) "afflicted" lycanthrope and you involuntarily transform every full moon until you become aware of your condition and transform voluntarily, which changes your alignment to the specific [[AlwaysChaoticEvil species alignment]]. If you're lucky you get bitten by a Werebear, they're Lawful Good.
* 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.
* ''TabletopGame/EclipsePhase'': Most strains of the Exsurgent virus turn the infected into a hideous alien monster or a brainwashed agent of the ETI, but the Watts-[=MacLeod=] strain only gives you a mental disorder or two, and PsychicPowers.

[[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.
* The Rot in ''VideoGame/{{Armello}}'', the King loses health each round from it, but any Hero infected with Rot doesn't really suffer any ill effects unless they fight someone else with Rot, in which case the one with a higher Rot score gains a bonus dice.
* ''Franchise/TheElderScrolls''
** The series has Vampirism, which comes in [[VampireVarietyPack many regional variations]] (which also explains the differences in the disease's characteristics between different games). One universal characteristic is that it renders the sufferer [[IdealIllnessImmunity immune to all other diseases]].
** Similarly, [[OurWerewolvesAreDifferent Lycanthropy]] also grants immunity to all other diseases. This makes Lycanthropy and Vampirism [[HybridOverkillAvoidance mutually exclusive]].
** ''[[VideoGame/TheElderScrollsIIIMorrowind Morrowind]]'':
*** The Corprus Disease, a "divine" disease created by [[BigBad Dagoth Ur]] using the [[CosmicKeystone Heart of Lorkhan]], makes the sufferer TheAgeless and immune to all other disease. It also unfortunately comes with a nasty case of BodyHorror and severe mental degradation. ([[PlayerCharacter The Nerevarine]] has the negative effects cured as part of the main quest, leaving only the positive ones.)
*** By making heavy use of exploits, it is possible to get both Lycanthropy and Vampirism at the same time. While the backwards vampire head on the werewolf body is funny to see, it has many [[GameBreakingBug nasty consequences]] that overshadow the benefits of either on their own.
* In ''[[VideoGame/DawnOfWar 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 ''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, adapting to the disease rather than fighting it off. 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.
* One increasingly common fan theory in ''Franchise/ResidentEvil'' is that constant ''light'' exposure to the series' various mutagenic viruses and substances has given most of the mainstay protagonists the superhuman strength, agility, and endurance they all seem to enjoy in the later games. The developers have [[ShrugOfGod neither strongly supported nor denied this.]]
** Directly confirmed to be the case for Sherry Birkin in ''VideoGame/ResidentEvil6'', however; after being exposed to, and then cured of the G-virus, her DNA has been modified just enough to give her amazing tissue-regenerative capability ''without'' turning her into a monster.
** BigBad Alexia Ashford from ''VideoGame/ResidentEvilCodeVeronica'' created the t-Veronica Virus believing it would give her godlike powers if her body was given time to assimilate the virus. [[spoiler: She was right, after fifteen years in cryogenic sleep Alexia was able to fly, use her combustible blood as a projectile, was almost invulnerable, and could psychically control an unknown amount of giant tentacles. It also made her grey and ant-like, but she was too crazy to care.]]
*** ''VideoGame/ResidentEvilTheDarksideChronicles'' shows further benefits of the t-Veronica virus. [[spoiler: Manuela Hidalgo was injected with the virus by her father to combat an unnamed illness and (unknowingly) had her infected organs regularly replaced for fifteen years. This cured her original illness and gave her the combustible blood power, though if she uses that power too much it can lead to self-immolation, and it disfigured her right arm.]]
* In ''VideoGame/MegaManX'', the Maverick Virus is this to Zero. In fact, according to WordOfGod, the virus had ironically fixed a programming bug in his "cognitive" system that made him very violent and disobedient, to the point where he performed a HeelFaceTurn soon after and joined the Maverick Hunters to face down other Mavericks. (The virus is supposed to make other robots violent by removing their inhibitions and their empathy).
* In ''VideoGame/DarkSoulsI'', the handful of Blighttown's original residents who are not infected by the disease are instead infested with Quelaag's spider eggs. Grotesque as this infestation is, it seems to have at least prevented the blight infection.
** There's also an interesting interaction with the Toxic effect and Dung Pies. In Blighttown, certain enemies shoot toxic darts at you. This inflicts a devastating damage over time effect that is usually a death sentence unless you have the curative item. There's an item called Dung Pies, however, which inflict a much less harmful "strain" of the same effect. If you're on the verge of being infected by Toxic, quickly using a Dung Pie will infect you with the less harmful strain, which is much easier to survive.
* In ''VideoGame/DarkestDungeon'', most diseases are crippling stat reductions that last until cured between quests. However, rabies actually gives a sizable damage bonus, while only reducing accuracy somewhat. It usually helps to leave rabies uncured on characters who are main damage dealers and already have dependable accuracy.
* The Armed Virus from the [[VideoGame/ValkyrieDriveBhikkhuni Valkyrie Drive]] [[Anime/ValkyrieDriveMermaid Project]] gives enhanced physical capabilities to its hosts and allows them to turn into powerful weapons when aroused.
* ''VideoGame/EternalSonata'': ALL diseases in Chopin's dream world. Any fatal illness gives the victim access to magic, leaving them pariahs but capable of surviving heavy-duty combat in lacy skirts.
* [[VideoGame/StarcraftII Stukov]] in ''VideoGame/HeroesOfTheStorm'' has the ability to spread a virus that heals his teammates.

[[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...
** [[http://www.scp-wiki.net/scp-016 SCP-016]] is a disease which [[AdaptiveAbility adapts]] the infected to any life threatening situation they are put in to keep them alive. For example, if an infected person is locked in a room being flooded with water, they may gain the ability to breath underwater or they may gain increased strength to help them break out of the room.
* On Website/{{Neopets}} it's not unheard of to deliberately infect a pet with a disease with an inexpensive cure to get rid of a different disease with a more expensive cure, since pets can only have one illness at a time.

[[folder:Web Comics]]
* In ''Webcomic/{{Sorcery101}}'', lycantropy is considered an infectious disease, usually spread through biting. There's even a (fairly simple) cure. In a classic case of CursedWithAwesome, however, it comes with a laundry-list of advantages - most notably rapid regeneration. Brad, who's a werewolf, was once offered a cushy job in return for biting his new boss's daughter... because she was currently dying from inoperable cancer, and he'd rather see her alive, if occasionally furry, than dead.
* The Vinn Parasite in ''Webcomic/{{Drive}}'' re-jiggers it's hosts DNA through a comorbid retrovirus. This results in [[MakeThemRot cytotoxic venom]], [[ShockAndAwe Bio-electrogenisis]], electropreception, and [[InvisibilityCloak chromatophores in the skin]]. The downside is that the parasite rejiggers the brain too, resulting in personality death.

[[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, what you're saying is...I'm indestructible.\\
'''Doctor:''' Oh, no, ''no.'' In fact, even a slight breeze could...\\
'''Mr. Burns:''' (walking off tenting his fingers) ''Indestructible.''

[[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 helped you cope with syphilis in later stages (by causing such dangerously hot fevers that the syphilis bacteria can't survive)--Dr. Julius Wagner-Jauregg even got a Nobel Prize for finding out during UsefulNotes/WorldWarI. 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 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 now has hookworms.
** [[https://www.youtube.com/watch?v=qD6fmHUXBEg SciShow has a video on the topic of the health benefits of worm infections.]] They recommend that you DontTryThisAtHome unless it is prescribed by a doctor.
** Tapeworms can sometimes cause weight loss because the parasite consumes part of the food you're ingesting. This led to a rumor that people have deliberately introduced tapeworm eggs into their systems as an alternative to dieting. These rumors are hard to verify, but it's not too big a stretch that '''some''' desperate souls may have tried it at some point.
* 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 [[SummonBiggerFish Cthulhu to fight Godzilla]].
*** [[http://scienceblog.cancerresearchuk.org/2013/06/25/no-doctors-did-not-inject-hiv-into-a-dying-girl-to-treat-her-cancer/ Actually]], they used HIV as a vector for gene therapy, the genetic material that makes HIV deadly was replaced with genes that modified a few T-cells, that were removed from the patient and infected in a petri dish before putting them back back, attack cancer cells.
*** There have also been a few cases where patients with AIDS and leukemia were seemingly cured of AIDS after receiving bone marrow transplants from people with a gene that makes them immune to HIV, but they later experienced a resurgence of the virus.
* A side effect of the so called ''[[https://en.wikipedia.org/wiki/Black_Death Black Death]]'' that ravaged throughout the 14th century was that the Leprosy bacteria was almost driven to extinction. The reason being that they preyed on the same victims and the former being much more vicious.
** Another upshot is through genetics: A portion of the population had genetics that resisted the Black Death, and this gene survived into successive generations (it's mostly in Eurasian stock, since that's where most of the plague was; Natives of America, sub-Saharan Africa, and the Pacific Islands and Australia don't have the gene in large numbers). Come the 20th century, it turns out that this gene also gives resistance to autoimmune diseases, including HIV.
** Related to all of this is ''[[https://en.wikipedia.org/wiki/Toxoplasmosis Toxoplasmosis]] [[PuppeteerParasite gondii]]'' - a parasitic infection that kills fetuses - but has the side effect of making the host [[CuteKitten adore cats]]. A ''big'' reason the Black Death [[DepopulationBomb depopulated]] Europe(anywhere from 75 million dead to '''200''' million dead!) is the superstition that cats are [[CatsAreMagic agents]] of [[CatsAreMean the devil]] - and were killed wholesale, leading to population explosions among rats, which did the same to fleas which carried plague bacteria. Humans infected with Toxoplasmosis [[CrazyCatLady befriended and protected cats]], which led to them surviving where their non-carrier neighbors died. One could imagine this resulted in numerous [[BurnTheWitch accusations of witchcraft]], but apparently disease killed cat-killers more effectively than TorchesAndPitchforks killed cat lovers. Hence, Toxoplasmosis infection indirectly protected carriers from ThePlague.
* 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. During the "tulip mania" striped tulips eventually reached the resale value of a small house.
* Sickle-cell anemia is a genetic disorder that causes your body to produce deformed red blood cells, which are less efficient at carrying oxygen. On the upside, the malformed cells also can't be infected by the malaria pathogen. Fortunately the heterozygous genotype doesn't have a notable effect on oxygen carrying yet still makes one immune to malaria.