Beneficial Disease

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.

— The Simpsons, Season 11 Episode 12: "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 — 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 cause 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. Compare Curse That Cures, 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 The Symbiote. Contrast Harmful Healing.

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Examples:

Advertising

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

Anime & Manga

• Baoh is about a man with a parasite which gives him super-powers but is still eventually fatal.
• The Eclipse Virus in Magical Record Lyrical Nanoha Force kills its victims horribly unless they drain life from others. It also grants them Anti-Magic and other powers to make the life draining easier.
• In the 25th episode of Mushishi, "Eye of Fortune, Eye of Misfortune", a mushi enters a blind woman's eye and allows her to see again. Of course, it also eventually allows her to see through objects and then into the future, before crawling out of her head and taking her eyes with it.
• A Tokyo Mew Mew episode features a Chimera Anima with a smelly fart attack. Mew Pudding just happened to have a cold that day, so...

Comic Books

• No less than three of Empowered's teammates got their powers from alien venereal diseases. This includes the man turned into a Blob Monster and another man who is a living robot.
• Zigzagged in the Futurama comic "A Cure for the Common Clod": everyone except Bender, Zoidberg, and Farnsworth gets a strange cold-like disease which makes them regress to their primal state. This isn't useful, but the usefulness comes from the fact that while everyone else sneezes up giant, Monstrous Germs just before they recover, the mutants sneeze up antibodies, so they deliberately infect the mutants so that their antibodies will fight the germs.
• Overlapping with Curse That Cures, the 1988 anthology Strip AIDS USA (a charity AIDS relief book) had a story where lycanthropy proves to be a great cure for HIV. Luckily, the recipient becomes a Friendly Neighborhood Werewolf.

Eastern European Animation

• The KikoRiki episode "The Chill" subverts this. Dokko catches a cold and Wally thinks it would be cool to catch one too because that way, he'd get treats. Despite doing everything possible, he fails to catch anything, but gets Krash and Chiko (who were trying to stop him) to feel sick. After he's informed that the two will have to get a lot of shots to get cured, Wally decides that he'd rather stay healthy.

Fan Works

• Downplayed in Anger Management: Haiku describes her cold negatively, but likes the pallor it gives her.
• The live vaccine against the Synthetic Hate Plague called Madsen's Hollow in Project Tatterdemalion is arguably this. It gives you fuzzy Combat Tentacles, retractable razor-sharp claws, technicolor tendrils in place of hair, Psychic Powers, the ability to sense and generate EMPs and use them to navigate and communicate not entirely unlike telepaths (sometimes to the point of Mental Fusion if the tendrils get close enough to interweave); Flash Step, an amazing healing factor, predatory instincts, and instinctively bond with other shinigami. 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 Harry Potter fanfic "Recnac Transfaerso", having cancer causes Harry to develop on-off superpowered magic.
• In the Harry Potter fanfic 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 The Rugrats' Crazy Fairy Tale, which is a fanfiction of Rugrats, the babies make a story in which "Tinker-Dil" has a cold, but it produces magic snot that makes Tommy able to become invisible.
• In Time Will Tell, Jorryn gets such a disease. The symptoms worsen until Jorryn faints in the USA. She wakes in the Shire (from The Lord of the Rings). This is a benefit because she can meet her favorite hobbits.

Films — Live-Action

• In 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. It seems like a fair trade-off, at first... but when the movie takes place, vampires outnumber humans 20 to 1, and vampire scientists are rushing to find a synthetic blood substitute before it's too late.
• 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. Reaper didn't know, but he sure benefited from it.
• In Phenomenon, George 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.
• According to the Backstory of Underworld (2003), 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 lycanthropy and 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.
• In the climax of World War Z, the protagonists discover that the zombies don't attack humans who are crippled or terminally ill, likening it to a predator avoiding eating diseased meat. So the plan becomes mass producing a fatal, yet curable, disease to give survivors so they can safely pass zombies into safe zones to be given antidotes.

Literature

• In the Cory Doctorow short story "0wnz0red", co-protagonist Liam is 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. It's a sexually transmissible wellness, dude. I've 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. It's going around, it's catching, you might already be a winner."
• Anita Blake: Lycanthropy 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.
• The Discworld series features the talking dog Gaspode, who notes that (like the Mr. Burns example discussed below) he suffers from so many diseases (including licky end, which should apparently only be contractable if one is a pregnant sheep) that he's only still alive because they're too busy fighting each other to focus on killing him.
• In The Expanse, Holden's chronic cancer makes him immune to the microbes colonizing the vitreus humors of every other human on Illus. Specifically his medication is toxic to them and administering it to everyone else causes their vision to clear up within an hour.
• Matou Kariya from Fate/Zero allows himself to be infected with "crest worms" that eat away at his body and will eventually kill him, but infuse him with enough magical power to summon and control Berserker, the strongest class of servant in the Grail War. It's his only hope of winning the Holy Grail and getting his wish granted.
• In The Fireman, a deadly spore causes Spontaneous Human Combustion. However, it is possible to control the spore, which in turn gives the host the power to create and control fire.
• Foundation's Fear: The childhood illness of Brain Fever is an inversion because catching "brain fever" makes one susceptible to R. Daneel's Psychic Powers. Hari Seldon had been lucky enough not to get sick at a young age, which means Daneel could not read his mind.
• Helliconia: Humans on Helliconia live with a virus that causes Bone Fever as the climate warms up and the Fat Death as it gets cold. Both plagues carry a terrible death toll, but the survivors are left with changed bodies that are suited to the coming season.
• In the Sesame Street book It's No Fun Being Sick, Herry thinks being sick should be useful because you get lots of attention, but when he does get sick, he feels bad and can't enjoy said attention.
• Known Space has the Tree-of-Life virus carried by a species of root vegetable native to the Pak homeworld. When Paks reach middle age, the roots become irresistible to them and they eat them, becoming infected with the virus and turning into super-intelligent and super strong Protectors single-mindedly dedicated to the preservation of their bloodlines. Since Earth is one of their lost colonies, humans can become Protectors as well.
• Melanie's Marvelous Measles tries to portray measles as a good thing because it grants you immunity, but it is portrayed unrealistically, as are vaccines, so the message kind of falls flat.
• In the 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 Zombie Apocalypse. A cancer- and cold-free Zombie Apocalypse, granted.
• Peeps: Carriers of the parasite enjoy super strength, enhanced senses, and extended lifespan. Unfortunately, most people who get infected become feral with a craving for blood and aversion to things they loved before their infection. And the parasite is very contagious, kissing is enough to infect someone.
• The Polity novel The Skinner is set on a Death World that has this gigantic leech whose bite carries a virus with interesting properties, "victims" become 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.
• Super Minion has two different examples. Benedicci is a clear-cut example, as it strengthens the immune system, modifies the brain to understand tinker tech, and in some cases also provides enhanced strength, reflexes, intelligence, or regeneration. In very rare cases these improvements can be strong enough to compete with supers and powerful mutants. Mutavus is less clear-cut, as it will frequently save the lives of people with medical conditions or life-threatening injuries and occasionally grant abilities on par with superpowers, but basically all results cause the patient to stand out in a crowd, and some mutations can put nearby people at risk or have other serious side effects.
• Tress of the Emerald Sea: The aether parasite in Crow's blood is constantly sucking the water from her body to send back to the prime aether on the moon, but it also uses its powers to protect her from any other threats so it can keep draining her.

Live-Action TV

• In the Buffy the Vampire Slayer episode "Killed by Death", Buffy's flu makes her sick, but also allows her to see the demon of the week, which had been killing sick children in the hospital for years.
• House has used one disease to cure another on more than one occasion, such as when the teenage faith healer whose touch seems to cure cancer turns out to have herpes, which attacks tumors.
• Kamen Rider Ex-Aid: Infection with the Bugster Virus risks the user being killed and replaced with a video game character if their stress rises too high, but with sufficient time to adapt to the infection, they can use a Transformation Trinket to gain various game-related superpowers without needing to undergo the normal compatibility-providing surgery, create new video games out of blank cartridges through force of will that they can use with the aforementioned trinket, or in extreme cases even manifest superpowers without a game. Adaptation to gain these powers takes over a decade of living a very low-stress lifestyle.
• 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.
• Possible Trope Codifier in the Red Dwarf episode "Quarantine", which features the crew discovering various positive viruses such as "Luck", "Joy", and "Sexual Magnetism". (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.)
• Sesame Street: In one episode, Telly gets something called "triangle-sneeze-itis", which makes him sneeze whenever a triangle is near. However, this is the only symptom so it's not like he has to rest or anything, and the sneezing comes in useful when they use it to find some valuable triangular objects.
• An episode of Smallville features a little boy with a brain tumor that gives 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.
• 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.
• Stargate SG-1: In "Upgrades", the characters receive a number of armbands that bestow superpowers on the wearers. They work by infecting the wearer with a nanovirus 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.
• Downplayed in an episode of Star Trek: Enterprise: Malcolm Reed gets a cold and while Hoshi says that he's lucky that his nose is too stuffy for him to smell a bad smell, she's mainly speaking in jest and his cold is mostly annoying to him.
• In Terra Nova, 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.

Tabletop Games

• Dungeons & Dragons:
  • Several "evil" diseases provide growing bonuses at the cost of penalties in other areas, in effect becoming a Deadly Upgrade. The players were quick to notice that the Prestige Class Cancer Mage from Book of Vile Darkness can adapt to diseases, hoarding bonuses while ignoring the penalties.
  • The same book include some evil spells with a disease component. This means the caster must be infected with the appropriate disease to cast them. Such spellcasters often use another spell called suspend disease to keep the infection from having any debilitating effect.
  • For that matter, Lycanthropy, at least in 3^rd Edition. 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 species alignment. If you're lucky you get bitten by a Werebear, they're Lawful Good.
  • The non-official Book of Erotic Fantasy introduces several sexually transmitted diseases. One of them, called "Whore's Delight", is only very mildly weakening, but has the weird side effect of making the sufferer's genitals excrete a paralyzing poison. This is used by dishonest prostitutes and their pimps to rob unwary costumers.
  • From the 3.5^th edition Monster Manual spin-off, Fiend Folio, among the Fiendish Symbionts are parasites from the lower planes such as the gutworms and soul ticks. They have drawbacks — like slowly driving good characters mad, and generally tempting non-evil ones into evil acts — but also confer some advantages.
    • Gutworms are similar to tapeworms, but they make their hosts globally healthier and can negate poison, although they also tire faster and need twice as much food as normal. A gutworm can also force the bearer into a Berserk Rage in combat.
    • Soul ticks slowly drain the blood of their hosts, but they confer greater power to their Black Magic, and surround them with a protective aura. The bearer may fail to cast good, chaos or positive-energy spells, though, and become vulnerable to holy power as if evil.
• Eclipse Phase: 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 Psychic Powers.
• FAPP (Not Safe for Work): Every living thing in the world of Jizzral, or those that enter it, are infected by a phenomenon called "the Fappening"; it causes those affected by the infestation to quickly recover from any injury at such a speed that they're practically immortal, receive abnormally increased sexual urges, and undergo random and strange physical mutations that often fall into the lewd area. There is no cure (at least, not any known ones), and the effect is spreading to many other worlds, and yet it does not seem malicious since the inhabitants of Jizzral consider it natural.
• In Warhammer, this is Nurgle's hat. Since he is a Plague Master 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 (and generally will) still be lethal to non-believers.
• While most werecreatures of Werewolf: The Apocalypse 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.

Video Games

• ARK: Survival Evolved: For a beneficial parasite example, an attached lamprey in Aberration saps your health but prevents radiation sickness. There are less painful ways to block radiation, but if you don't have those and can heal off the damage...
• The Rot in Armello; any hero infected with severe rot (especially the king) loses one HP on odd-numbered rounds (morning) and becomes susceptible to the cleansing magics of the Wyld, but at high levels, Rot can be used to overpower the Banes, other Rot players, and the King himself.^note In a Rot battle, whoever has a higher Rot stat gains dice equal to their opponent's Rot stat.
• Vampirism in Castlevania 64 might take away your primary weapon and ability to recover health by eating food, but it also makes you unable to die. You still take damage, but you keep going even when your health reaches zero. As long as you keep in mind that you'll get a Non-Standard Game Over if you remain in vamp status until next midnight, it can be situationally useful if you keep an eye on the clock and keep a Purifying Crystal on hand. It best comes into use when forced to navigate the odious Garden Maze and its invincible Demonic Spiders, since you handily fight a vampire immediately before entering the maze...
• In Darkest Dungeon, 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 have consistent accuracy.
• In Dark Souls, 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 suppressed or cured the symptoms of the blight.
• 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.
• The protagonist of Deep Fear is immune to The Virus because he's got a cold.
• The Elder Scrolls:
  • The series has Vampirism, which comes in 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 immune to all other diseases. In addition, the Skyrim variation of the disease, Sanguinare Vampiris, bestows the Vampiric Drain spell.
  • Similarly, Lycanthropy also grants immunity to all other diseases. This makes Lycanthropy and Vampirism mutually exclusive.
  • Morrowind:
    • The Corprus Disease, a "divine" disease created by Dagoth Ur using the Heart of Lorkhan, makes the sufferer The Ageless and immune to all other disease. It also, unfortunately, comes with a nasty case of Body Horror and severe mental degradation. 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 nasty consequences that overshadow the benefits of either on their own.
  • In Skyrim, the Afflicted are infected with a disease that gives victims the ability to spray acid out of their mouths. Even though it is supposed to be an infectious disease, it can't be spread to the player.
• Eternal Sonata: 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.
• In Fallout, the US government tried to engineer one as part of the Pan-Immunity Virion Project, intending to create an actively evolving antidote to the New Plague ravaging the world and to immunize the US against bio-warfare attacks by China. When they discovered the potency of its Mutagenic Goo properties, they scrapped that project, renamed it the Forced Evolutionary Virus, and tried to use it as a Super Serum. This... did not work out so well, leaving the fanbase split on exactly how extensive a role FEV played in creating the various Nuclear Mutant creatures of post-apocalyptic America.
• Griftlands: The Bog Parasites^note acquired on Day 2 of Rook's campaign or brawl are generally a harmful nuisance, as they take up a card in your decks and usually deal 1 damage when they're drawn. However, draw and/or use them enough times and they 'Hatch' into special-effect cards.
• In Heroes of the Storm, Stukov has the ability to spread a virus that heals his teammates.
• In Lionheart: Legacy of the Crusader, the Disease Ward perk grants your character passive healing when sick.
• This is part of the quarians' backstory in Mass Effect. 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.
• In Mega Man X, the Maverick Virus is this to Zero. According to Word of God, the virus had ironically fixed a programming bug in his "cognitive" system that made him very violent and disobedient, to the point that he performed a Heel–Face Turn 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.) This is even taken into gameplay in Mega Man X5, where Zero getting infected by the floating viruses that roam in some stages makes him temporarily invincible (while X instead loses part of his health).
• Pokémon: From Generation II onward, the main 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 Pokémon 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.
• Lili from Psychonauts gets a cold that stuffs up her nose just before she's kidnapped. Thankfully, her stuffed nose keeps her from breathing in Dr. Loboto's special pepper, so she doesn't sneeze her brain out like the other kids.
• One increasingly common fan theory in Resident Evil 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 neither strongly supported nor denied this.
  • Directly confirmed to be the case for Sherry Birkin in Resident Evil 6, however; after being exposed to and then cured of the G-virus back in Resident Evil 2, her DNA has been modified just enough to give her amazing tissue-regenerative capability without turning her into a monster the way it did her father.
  • Big Bad Alexia Ashford from Resident Evil – Code: Veronica believed the t-Veronica Virus she created would give her godlike powers if her body was given time to assimilate the virus. She was right — after fifteen years in cryogenic sleep, Alexia is able to fly, use her combustible blood as a projectile, is almost invulnerable, and can psychically control an unknown number of giant tentacles. It also makes her grey and ant-like, but she's too crazy to care.
  • Resident Evil: The Darkside Chronicles shows further benefits of the t-Veronica virus. 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 using that power too much can lead to self-immolation, and it disfigured her right arm.
• There are a couple of these in Space Station 13, such as Owns Syndrome, which heals, confers stun recovery, and gives free sunglasses.
• Appears several times in the Trauma Center (Atlus) 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, 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 Armed Virus from Valkyrie Drive -Bhikkhuni- gives enhanced physical capabilities to its hosts and allows them to turn into powerful weapons when aroused.

Webcomics

• The Vinn Parasite in Drive (Dave Kellett) re-jiggers its host's DNA through a comorbid retrovirus. This results in cytotoxic venom, bio-electrogenisis, electroreception, and chromatophores in the skin. The downside is that the parasite rejiggers the brain and germ cells too, resulting in personality death and sterilization.
• Manly Guys Doing Manly Things: According to Commander Badass, everyone in the future has cancer. It's such an integral part of their bodies that future humans have formed a symbiotic relationship with it to the point that cancer has stopped being dangerous and instead functions as a minor Healing Factor.
• In Sorcery 101, lycanthropy is considered an infectious disease, usually spread through biting. There's even a (fairly simple) cure. In a classic case of Cursed with Awesome, 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.
• xkcd strip #938 comments on a medical trial where the doctors tried to use HIV to treat leukemia.

Websites

• In 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.
• SCP Foundation:
  • 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...
  • SCP-016 is a disease which 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 breathe underwater, or they may gain increased strength to help them break out of the room.

Western Animation

• One episode of Ben 10 has Ben get sick with a cold, which causes problems when he transforms into aliens because he still has the cold when transformed and it has weird effects on his alien forms (Wildmutt becomes blind due to it clogging up his sensory organs and Fourarms is weakened and smells really bad), but when he transforms into Heatblast, it causes his fire powers to be replaced with ice powers, which prove useful.
• In the Betty Boop episode "Betty Boop's Kerchoo", Betty Boop has "a cold in [her] nose", which seems to make her win because when she sneezes, it pushes her car along (although having a car with the ability to go over other cars when a lever is pushed helps, too).
• In the Futurama episode "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. However, Fry got rid of them after he realized Leela only loved him because they changed him.
• Played with in the My Little Pony: Friendship Is Magic episode "The Cutie Pox". The titular illness makes Apple Bloom able to do amazing things like tame lions and do complex equations, and it seemingly prevents her from getting tired as she tap-dances all night without seeming tired even afterwards. However, it also forces her to do those things, making day-to-day life difficult.
• Downplayed the Pinky Dinky Doo episode "Polka-Dot Pox". While Pinky and her classmates do have to stay home from school due to having the titular disease, its only symptom is polka dots on the face, and it gives them the excuse to throw a "polka-dot party".
• The Simpsons: In the episode "The Mansion Family", it turns out that Mr. Burns is 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.

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 World War I. 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 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.
  • SciShow has a video on the topic of the health benefits of worm infections. They recommend that you Don't Try This at Home unless it is prescribed by a doctor. There are plenty of medications that do the same thing and usually are safer so for most people medication is a better choice.
  • 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 cure cancer with AIDS on purpose, which in medical terms is somewhat like summoning Cthulhu to fight Godzilla.
    • 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, to 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. (The mutation blocks the path that most HIV strains use to attack cells. The virus could have mutated to attack using a different path, or they were already infected with an HIV strain that used the other path in addition to the common type of HIV.)
• A side effect of the so called Black Death that ravaged throughout the 14th century was that the bacteria responsible for Leprosy was almost driven to extinction. The reason being that since victims of the two diseases were almost always housed together, the plague would infect those suffering from leprosy as well and kill the host before it could spread further.
  • 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.
• 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" among the Dutch in the 17th century, striped tulips eventually reached the resale value of a small house.
• Myiasis, the infestation of an animal (human or otherwise) by maggots, can be beneficial when it occurs in a wound, as the larvae feed on dead tissue and reduce the chances of infection. This can occur with natural infestation of wounds or be done deliberately as maggot therapy. This has recently become more popular as maggots can deal with many of the bacteria that are resistant to antibiotics. However, this is only beneficial with specific species that prefer dead tissue over living tissue, and even these species will be harmful if too many maggots are used.
• Doctors are experimenting with using viruses called bacteriophages to treat antibiotic resistant bacteria. These viruses are specialized for infecting specific bacteria and are not capable of infecting human cells (so it's really a subversion; these viruses are making the bacteria sick, not you, and it is very much a detrimental disease for the bacteria). Bacteria can develop resistances to the viruses, but by doing so lose their resistance to antibiotics.
• There exists a virus that infects the Marine Flagellate Cafeteria roenbergensis (simply called the Cafeteria roenbergensis virus or crov for short) that's 100% fatal to the Flagellate and can wipeout entire populations of them. The exception happens when it's also infected by the Virophage Mavirus, who will hijack the crov virus's replication process and make it create more mavirus particles instead. This will still kill the flagellate eventually but it also means that less of them overall will get infected by crov by reducing the number of viruses in a population.
• There is some evidence that herpes virus infections such as chickenpox, which stay in the host for life in latent form once the initial symptoms subside, may actually help protect the host by making their immune system more responsive to other infections. Herpes viruses are so common that almost everyone gets infected with one during their lifetime and so the human immune system has adapted to the body having them.
• Botrytis cinerea is a fungus that affects many plant species. While the resulting fungal infection can destroy crops, if wine grapes are infected under the right conditions, it becomes what winemakers call "Noble Rot", and results in highly desirable dessert wine.

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