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- In Higurashi: When They Cry, Rika. She's immune to Hinamizawa Syndrome and medicine is derived from her blood.
- Marco and Zeus are immune to the Medusa Virus in King of Thorn because they are too confident in themselves for it to find a crack in their psyches to infect.
- This is part of the premise of Double Arts, in which the world has been decimated by a mysterious plague (a plague with no permanent cure). While there is a small percentage of young women who possess a tolerance to the disease, it is only the protagonist, a young man named Kiri Luchile, who is truly immune to its effects and even has the capability to nullify the disease when it reaches its final stage; by physically touching the infected person, Kiri can effectively halt the progression of the plague and stave off the infected person's death, but only so long as he holds on to them. This is a major plot point.
- In Casshern Sins, Casshern and Friender are the only ones who don't succumb to the Ruin, which causes the normally immortal robots to rust and decay much faster than in Real Life. Casshern is immune because he caused the Ruin and is apparently something along the lines of The Punishment. The source of Friender's immunity isn't spelled out explicitly, but it's implied to have come from eating some of Casshern's flesh.
- In Anatolia Story, Yuri proves to be immune to the plague ravaging the Hittite Empire when she spends an extended amount of time tending to the sick. Given that she's from modern times, Yuri believes her vaccinations have granted her immunity, while the people of the ancient time period just take this as further proof that Yuri is blessed by the gods. (A bit of Artistic License there: the historical plague that swept through Anatolia is generally agreed to have been tularemia, which is not a general vaccination and is only administered to high-risk groups due to dangerous side effects. Not to mention that one of the historical figures Yuri is based on, Gassulawiya, did die to the plague.)
- World's End Harem: Reito Mizuhara is immune to a virus that only targets males and has wiped them out of the face of earth in 2040 to 2045, the women theorizes that a combination of him contracting multiple sclerosis, going into cyostatis in hopes they would find a cure during his cold sleep, and consequent cure during said sleep made him immune to the virus; now possessing valuable genes that the women compete over to birth a new human race.
- Saul and his mother Tanya from We're Alive are believed to be immune to the zombie outbreak after several close calls. This is pretty much confirmed in the final season when Saul is bitten directly on the neck by a zombie and doesn't turn. In the epilogue, many people believe that Saul and Lizzy's son, Nicolas, may also be immune, but he hasn't been exposed so it's unknown whether he is or not.
- The reason everybody was looking for Ray Palmer in Countdown to Final Crisis
- Also a plot point in Maximum Clonage arc of Spider-Man, where one person survived the test run of the Jackal's virus
- The Batman: Contagion story had a series of these, each typically revealing that there was one other person just before dying (and rendering the antibodies in their blood immediately useless).
- Yorick and Ampersand, the only males to survive the Gendercide in Y: The Last Man. Or so they thought.
- In All-Star Superman, Steve Lombard was immune to being turned into a Bizarro. He smugly asks Supes to scan him with his X-Ray Vision and create the antidote, but Superman answers, embarrassed, "I don't think I can recommend your performance enhancers to the rest of the population".
- Alice in Resident Evil: Extinction. It doesn't have any actual bearing on the plot, mind; it's just exposited by the White Queen at the end.
- Given Umbrella bonded her with the T-Virus at the end of Resident Evil: Apocalypse, it makes sense that she is immune. She's already infected.
- The survivors of the plague in Doomsday, with Cally in particular serving the role of "make a cure from her blood".
- The protagonist of I Am Legend is one of the few humans that are immune to the disease. He spend the next three years trying to synthesise a cure out of his blood for the humans that instead mutated into the feral Darkstalkers.
- In the Will Smith version he's not the only one, in every other adaptation he is the last uninfected human, ever.
- Not true immunity, but in 28 Weeks Later both the wife and the son are infected by the Rage Virus, but show no symptoms, instead becoming carriers. It's implied that it has something to do with the fact they have heterochromic eyes.
- In Skinwalkers the key to curing lycanthropy are the antigens in a half-werewolf, half-human boy's blood.
- Joanie in Warning Sign is immune to the bio-engineered Hate Plague because she's the only pregnant woman in the quarantined facility; the special hormones in her blood is what keeps her safe and is the basis of the cure. What Do You Mean, It's Not Symbolic??
- In the So Bad, It's Good movie Robot Monster, the only people who are left alive after the alien invasion are the friends and family of a middle aged scientist who'd dosed them all with a cure-all serum. (Initially though, the scientist thinks it's a protective electric barrier he erected that staved off the alien's attack.)
- In Contagion Mitch Emhoff turns out to be naturally immune.
- The doctor's wife in Blindness is the only one whose eyesight is retained in the mysterious blinding epidemic.
- Of the few surviving humans in Dawn of the Planet of the Apes, it's stated that most are immune to ALZ-113/Simian Flu.
- The Perfect Dark: Initial Vector novel.
- A big focus of The Andromeda Strain was finding out why two people were immune to the disease when everyone else died
- What made it interesting was that they were immune for similar, yet different reasons. The old man was an alcoholic, so his blood pH was acidic, and the baby was hyperventilating through crying, which made his blood pH alkaline. Turns out the Andromeda Strain could only survive within a narrow pH range.
- In The Changeling Plague, IdahoBlue was one of these for a previous disease epidemic (though not the titular one)
- In Chasm City of Alastair Reynolds' Revelation Space universe, the cure for the Melding Plague, called Dream Fuel, turns out to be blood harvested from an alien with natural immunity
- Dr. Robert Neville in virtually every adaptation of I Am Legend.
- The thief in Brian Evenson's short story Fugue State, who breaks into and loots quarantined apartments.
- José Saramago's Blindness follows the one woman immune to the plague of blindness.
- In Oryx and Crake, Snowman aka Jimmy believes that he is the only human left in the world after a man-made hemorrhagic virus destroys humanity in a matter of weeks. He inadvertently gained immunity some time before the outbreak of the pandemic via a vaccine created by his friend Crake.
- Everyone who lives through the first act of The Stand.
- Every character in George R. Stewart's Earth Abides is immune to the plague for one reason or another - which is only reasonable, since all the non-immunes die within the first few pages.
- In Diario de un Zombi Paula is immune to the zombie virus, a major plot point since finding out why can save the human race.
- In The Lord of the Rings, Tom Bombadil is the only being completely unaffected by the One Ring (it cannot even make him vanish when he puts it on his finger), implied to be because there is nothing he desires in the world that the Ring can tempt him with. The Council of Elrond considers giving the Ring into his keeping because of this, but the problem is that the very cause of this immunity means he wouldn't be a good guardian—he wouldn't be able to appreciate how important the Ring is and would casually toss it aside and forget about it.
- As Tom doesn't seem to fit anywhere into Middle-Earth's greater cosmology (only really becomes apparent if you read The Silmarillion, where the connectedness of everything else is laid out), it's possible that the Ring doesn't work on him because he is somehow outside its influence.
- In The Death Cure, it is revealed that most of the kids who were part of WICKED's Trials, along with a very small percentage of the world's population called "Immunes", are immune to the Flare. WICKED took them from their families because they wanted to study their brains in the hope that this would eventually lead to a cure. The few among them who are not immune were chosen as control subjects.
- In The Divergent Trilogy, Tris is able to resist serums, like most Divergents. She is notably able to fight off Candor's truth serum in Insurgent, and cannot be killed by the Bureau's death serum in Allegiant. At multiple points, it is mentioned that her ability to resist is beyond that of most Divergents.
- In Scott Westerfeld's Peeps Carriers for some unknown reason get the benefits of the parasite that causes "vampirism" while still managing to remain sane. Night vision, super strength, and long lifespans without the urge to commit cannibalism (just a craving for rare meat and a high libido) or aversion to everything they loved before infection. Unfortunately they can transmit the parasite with as little as a kiss, those who work as hunters usually start out with their exes.
Live Action Television
- There was a Star Trek: Voyager episode where B'Elanna's unborn baby provided the cure to a Klingon disease
- Heroes: Mohinder's blood carries the cure for the Shanti virus
- A big plot point in the second season of Dark Angel and the three follow-up novels. The Breeding Cult plan to unleash a disease that only they will survive, but Max turns out to possess total immunity
- An episode of The X-Files dealt with an oil rig worker who was immune to the alien black oil.
- An episode of Terminator: The Sarah Connor Chronicles revolved around them having to save a woman who will be naturally immune to a disease Skynet engineers to wipe out the Resistance.
- In the BBC series Survivors (the 2008 remake/"reimagining"), the last remnants of humanity are those who never caught the "European Flu" or who are naturally immune to it. Abby is the only person to ever develop symptoms but then recover, which causes her to get abducted in the season one finale.
- The Stargate SG-1 episode "The Broca Divide", where Dr. Fraiser is immune to the virus due to the strong antihistamines she takes for allergies.
- Daniel Jackson was temporarily immune for the same reasons until he was trapped off-world and no longer had access to his medication.
- Teen Wolf's Lydia Martin is the only known person who is immune to the bite of a werewolf and the venom of a Kanima. The reason behind this is currently unknown, but it's implied to have something to do with her unexplained ability to sense the supernatural.
- As of Season 3, we know it's because she is a Waling Woman, otherwise known as a banshee.
- Sam Winchester in Supernatural is immune to the Croatoan virus.
- This is part of The 100's backstory. When the nuclear bombs fell, certain people had a natural resistance to radiation, allowing them to survive while the rest of humanity died out. Their radiation immunity was passed on to their descendants, who became the Grounders.
- On My Name Is Earl, the "Witch Lady" tried to drug Catalina's tea, with the intent of storing her in the basement along with all the others she intended to kill for bullying her. Catalina gets down into the basement, but explains that she wasn't affected by the drug because so many men have tried to slip her date-rape drugs in the past that she's immune to them. She then explains that she only pretended to be knocked out and allowed herself to be bagged and tossed in the basement is that she felt sorry for the Witch Lady.
- The Last Ship: Most of humanity dies out as a result of a Synthetic Plague, with the eponymous ship going on a mission to develop a cure. In the second season it becomes apparent that a lot more people have a natural immunity than previously thought (around 5% of the population). They start a cult that portrays them as the "chosen people" and set out to kill all non-immunes, led by two immune British brothers who were the only survivors of the nuclear submarine they both served in.
- In Magic: The Gathering, several members of the Mirran Resistance against New Phyrexia are immune to Phyresis, the most notable one being the Sylvok outcast Melira, who was born without any metal in her body, as most beings on Mirrodin were (and Phyresis affects metal).
- An expensive (and frequently banned) merit in Werewolf: The Apocalypse makes the character immune to contamination by Wyrm taint. The book notes that this will probably lead to some dangerous missions.
- One minor NPC in Ravenloft's domain of Dementlieu is a man who unknowingly inherited a ring of mind shielding. The mind-controlling villain who secretly controls that nation is aware this man is immune to his powers, and is determined to find out why.
- In the post-apocalyptic "economic horror" RPG Red Markets some people are immune to the zombie virus, and others, called "latents", become carriers after being bitten and become zombies after dying for some other reason. Immunity or latency can be established at character creation or discovered after getting bitten and making a critical success or critical failure on the fort save, respectively.
- As an Anti-Frustration Feature, this is usually the case with anything involving zombies or similar creatures in video games. There are games where you have to manage your infection level, but they tend to be rare (examples include Extermination or Resident Evil Outbreak.)
- It's used as a plot point in both Dead Rising games. Some people are more resistant to the zombie virus than others; such resistants include Frank West and Tyrone King. Chuck's apparent immunity, while unexplained, is a natural step up from that.
- In Infected, the main character is not only immune against the zombies, but his/her blood is only thing (short of a nuke) that can kill the zombies. So a scientist makes a gun that shoots his/her blood at zombies, gives them a gun, and tells them to go clean up New York City.
- Sgt. Johnson was immune to infection by the Flood, either due to "Boren's Syndrome" caused by "overexposure to plasma grenade radiation", or the real reason hidden by the previous cover-up - the chemicals that were used to augment Johnson into a Spartan-I.
- Due to the physiology of Hunters (they're worm colonies with no central nervous systems), they are immune to direct Flood infection.
- In Halo 4: the Master Chief is exposed to the Libarian's memories, rendering him immune to the Composer.
- The survivors in Left 4 Dead.
- Some of the graffiti in the second game and radio chatter near the Bridge Finale suggests the survivors are carriers of the Infection. A supplementary comic explicitly explains that the immunity is genetic, and confirms the carrier part: "immunity" keeps people with it from turning, but they can still transmit the virus to others.
- Not so much a plague as a curse, but Adell in Disgaea 2 is the only one who doesn't turn demonic under Zenon's influence. Turns out it was because Adell was a demon to begin with. He never discovers this himself, but it becomes quite obvious to the player.
- Carrying into the fourth game, Artina, a.k.a. Vulcanus shows absolutely no signs of A-Virus corruption during Chapter 6, and took a shot meant for Valvatores at the end of Chapter 5. The blood of angels like her is a panacea for all forms of disease, the A-Virus included. While Valvatores resists through sheer willpower, or so he claims, after winning Chapter 6, she issues blood samples to the demons both to treat the pandemic and simultaneously fill Celestia's coffers.
- The baby in The Pitt DLC of Fallout 3 is immune to harmful mutations caused by nuclear radiation. She is also resistant to 'Troglodyte Degeneration Contagion', a condition caused by radiation and industrial toxins that turns humans into sub-human monsters.
- Tsugumi of Ever17, whose blood provides the cure for Tief Blau.
- Samus in Metroid: Fusion becomes immune to the X parasite in the opening. She got infected and some scientists try an experimental cure: cells from the (now dead) baby Metroid. It works, and she gets sent to exterminate the X in the BSL station. The X get smart, though, and cold X start showing up at a later point in the game (this is because Samus inherited the Metroid's weakness to cold as well as its ability to absorb X). It takes an armor upgrade later on to allow Samus to absorb cold X.
- 1213: the protagonist is immune to any disease or poison, which sets up the horrifying ending.
- Reimi in Star Ocean: The Last Hope. This is because as a Seed of Hope, she is genetically engineered for survival on new and unknown worlds. Unfortunately, she ended up with a case of Survivor Guilt when a radiation leak killed off all of her friends except for her.
- The four survivors in Dead Island are immune to the virus that turns the entire island of Banoi into zombies. As with Left 4 Dead above, it turns out they're not exactly "immune" so much as they are "asymptomatic carriers".
- In Ninja Blade, parasitic hookworms known as "Alpha-worms" end up infesting the whole city and turning them into monsters...except for the ninja protagonist Ken Ogawa, who for some reason seems to be the only one immune to being infested, and can thus safely battle all the enemies. This turns out to be because he has toxic blood in him, which is deadly to any parasites that would dare try to infest him. And he has that toxic blood because his father, Kanbe, has it too. So the immunity is genetic.
- Ellie in The Last of Us. Technically they are Infected, but a random mutation rendered their fungal growths harmless. At the end of the game, the Fireflies attempt to create a vaccine from her, even though the process will kill her. Unable to accept this, Joel instead kills his way through the Fireflies and escapes with her, thus possibly dooming humanity.
- In Zombie Exodus you find out Devlin was bitten but didn't turn.
- In Dead Rising 3, protagonist Nick is immune to zombification. This is because Carlito inoculated Nick with the cure instead of the zombie virus like the other orphans to ensure that his Zombie Apocalypse would not be unstoppable.
- The Resident Evil series has several notable examples:
- It was eventually explained via subtle Retcon that the T-virus and its variants weren't 100% effective and roughly 10% of humanity was resistant to it to some degree, in comparison to the original games running off the assumption that "your character is just so badass that they never got hit". Furthermore, Umbrella has worked on various antiviruses to the T-virus, most notably seen in Resident Evil 3: Nemesis.
- Sherry Birkin is more or less kidnapped by the US Government and put under constant surveillance because, due to the events of Resident Evil 2, she is the sole known source of both the G-Virus and the Devil Vaccine that cured it. In Resident Evil 6, it's revealed that the Devil only suppressed G and allowed her body to assimilate it, giving her a powerful Healing Factor without losing her mind or turning into a mutant horror.
- Jill Valentine is implied to be immune to the T-Virus after being cured of her infection in Resident Evil 3. This is confirmed in Resident Evil 5, when it's noted that after she got kidnapped by Albert Wesker, she turned out to be useless as a test-bed for T-Viral experiments due to her innoculation back in 3. However, in the same game, it also works against her, as the antibodies in her blood were used to improve the Ouroboros virus and this made Wesker decide to brainwash her into serving him as The Dragon rather than just experimenting on her.
- Jake Muller is completely immune to the effects of the C-Virus because he is Albert Wesker's son and was conceived after Wesker was juiced up on the virus that gave him superpowers.
- Sergei Vladimir is a variant; he's one of the one-in-ten-million who has "total genetic compatibility" with the T-Virus and all its strains, which means that infection doesn't turn him into a Technically Living Zombie, but one of Umbrella's ultimate Super Soldiers, the Tyrants.
- In Resident Evil 7: biohazard, both Ethan Winters and Zoe Baker are this to a degree, downplaying the trope. Ethan is heavily resistant to the Mind Control aspect of the mutamycete's fungal infection, and so he doesn't start to begin suffering it until the final minutes of the game. Zoe, meanwhile, seems to be completely immune to the Mind Control side of the infection, which is why she is The Unfavorite amongst her infected kin, but this doesn't protect her from Eveline's ability to simply kill off those who are infected with her fungus, even after taking the fungicidal serum.
- Eve in Mass Effect 3 wasn't originally unaffected by the genophage, but was cured by Maelon in Mass Effect 2, and is now the most important person when it comes to finding a cure.
- BlazBlue actually downplays this trope. Seithr corruption is a perpetual threat in the world due to the omnipresence of ars magus, but seeing as how it is necessary to maintain the current state of the world, nobody sees a problem with it. That said, different individuals process seithr at different rates, so some folks show corruption effects sooner than others. Where this trope comes in is a bit of foreshadowing in Arcade mode, where precisely one character has their erosion rate measured, and is noteworthy of having it being extremely low: Makoto Nanaya, at 0.2%.
Hakumen: So there are still souls in this world as pure as yours. I suggest you cherish this rare commodity, for the sake of us all...
- It's not really brought up in the game, but some of the in-game documents in Dishonored hint that all Serkonans are immune to the Rat Plague because they are the descendants of survivors of a Rat Plague outbreak on Serkonan who passed on their genetic immunity to their offspring. This would explain why Corvo and Daud, both Serkonans, never contract the Plague despite being exposed to it so often.
- Dragon Age
- Meta-example: In Dragon Age: Origins, the entire party is this to the darkspawn taint. Alistair and the player character already have the taint by virtue of being Grey Wardens, but the others in the party develop an immunity to it purely as a gameplay necessity. It was originally intended that the party members could be put through the Joining ritual to become Grey Wardens, as they would otherwise be dying from the taint toward the end of the game, but this was jettisoned for plot and time reasons. Their immunity is not mentioned in the game.
- This actually serves as a minor plot point in Dragon Age: Inquisition. A magister researching Leliana, a returning character and former Origins companion discovers her to have a higher resistance to the taint than any known subject.
- Members of the Seekers of Truth, essentially Templar Internal Affairs, are immune to mind control and Demonic Possession. It is a benefit of the same initiation ritual that allows them to use special anti-magic powers without lyrium. Seekers are made Tranquil and then immediately cured by having a Spirit of Faith touch the initiate's mind. If successful, the initiate gains the powers and immunity of a Seeker. If unsuccessful, the initiate is left permanently Tranquil. At some point, a mage attempted to join the Seekers, seeing it as a solution to fact that Mages are at much higher risk of possession. He failed, and accidentally discovered that Tranquil mages can no longer use magic and become invisible to demons. Things spiraled downward from there over the next nine-hundred years.
- An odd twist can be seen in World of Warcraft; Anyone under the Worgen Curse is immune to the plague of undeath.
- In Deus Ex, JC Denton tests positive for Gray Death, but shows no signs of infection. It's because the Gray Death is a artificially created nano-virus and JC's nano-augs use the same hardware architecture. He's already eyeballs-deep in such nannites. Presumably, his brother Paul is similarly (not) affected.
- The Elder Scrolls III: Morrowind: It is not given much attention, but this is one of the reasons why the Nerevarine is sent alone into Red Mountain, aside from the prophetic and infiltration aspects — earlier in the main quest the Nerevarine gets infected with Corprus and then cured of the negative sides, which leaves them immune to all diseases, including Blight and Corprus. Since Red Mountain is the source of the Blight and Corprus...
- In Wandering Ones, this is the reason Mavon's plague didn't completely sterilize the Earth's human population.
- The Plague in Stand Still, Stay Silent attacks all mammals, but in every species, some percent is immune. For humans, in Year 90 it's about 11% on Iceland and 48% in the rest of Known World. All cats and non-mammalian species are immune as well.
- The trope gets some degree of deconstruction, as the setting regularly shows the reader that the Plague Zombie monsters are still dangerous to those who are immune. In the prologue, immune non-combatants point out that they would die if they went outside the safe areas. Later in the story, an immune character who took a Plague Zombie bite in place of someone who isn't immune develops an infection due to the fact that she got bitten by another (sort of) living being, which carries the same risks as being bitten by an animal in real life, regardless of her incapacity to catch The Plague.
- Uncreation is about an entire knightly order of them, who stage crusades in the plagued lands on the behalf of The Empire.
- In Manly Guys Doing Manly Things, Canadian healthcare has rendered Canadian Guy immune to Nomura Syndrome. They utilize this to get everyone else cured.
- In Endtown there are a few people who seem immune to the mutation virus. The Topsiders refer to them as "Typhoid Mary's" since even if they aren't mutated, still carry the virus, and do even more horrific things to them than normal mutants, who are simply disintegrated or turned into Wetware CPUs.
- A Simpsons Halloween special had Bart immune to the zombifying effects of the Krusty Burger. The survivors of the outbreak wanted to eat Bart, but they compromised by having Bart taking a bath with their food.
- The lice episode of Invader Zim. Zim (and Ms. Bitters, actually) are immune to the lice infestation. This leads to Zim being studied, and guess what? His skin, it destroys the lice! This lead to the creation of a Skin Gun to take down the "lice Queen". They do eradicate the Queen at least... except Ms. Bitters just has a higher scratch threshold than most, and was infected.
- In the television version of the X-Men "Legacy Virus" storyline, Cable came back in time to ensure that Wolverine would be infected with the titular virus, because his healing factor would both render him immune and lead to the development of a cure.
- In South Park, Cartman and Kyle get infected by HIV, and Cartman vows to find a cure. They go to Magic Johnson's house to figure out why he recovered from HIV. They notice that his bedroom is full of piles of money, because he doesn't trust banks. Sure enough, injecting shredded money into their veins cures them. Then Africa is informed of the good news, all they have to do is roll around in all their money.
- On the SpongeBob SquarePants Christmas Special "Its A Sponge Bob Christmas", Plankton uses jerktonium to make everyone into a jerk. SpongeBob, however, turns out to be immune to its effects due to his 100% pure heart.
- Dairy farmers in the 18th century were found to be immune to smallpox, due to exposure to cowpox, which is caused by a very similar, closely-related virus but is far less virulent.
- The discovery that milkmaids had unusually low occurrence of smallpox led to Edward Jenner's invention of vaccination. Historically, inoculation for smallpox involved taking ground-up smallpox scabs and putting them in a cut on the hand (or some such), which did usually lead to immunity but would also typically lead to a mild case of smallpox, and in over 20% of cases full-blown smallpox (John Adams' younger son Charles had such an episode 1776; the process of inoculation is shown in all its graphic detail in John Adams) and in 2–3% of cases would lead to death (which is substantially better than the death rate for smallpox — 20–60% — which is why people bothered in the first place). Jenner's discovery led to a similar process being used with cowpox, which would almost never kill and which typically only caused a bit of inconvenience.
- The very word vaccination comes from vaccus, Latin for "cow", in honor of Jenner's discovery.
- Individuals with the Delta 32 Mutation are immune or highly resistant to HIV infection. Historical and family records indicate that the ancestors of these individuals also had a higher survival rate during the Black Plague.
- While the Delta 32 mutation is the most famous mutation to grant aids resistance, people with this gene are actually the minority of long-term nonprogressor individuals with HIV. Other genes that assist in stopping aids progression are various HLA types: HLA-B5701, HLA-B5703, and HLA-B2705.
- Individuals with these genes are usually not totally immune to HIV, but instead show the ability to exert long term control over the virus without medication, and may be carriers in some cases.
- Indigenous populations in areas where malaria is a concern are often prone to sickle-cell anaemia, a genetic disorder characterized by malformed sickle-shaped blood cells. Two copies of the defective gene sickle all blood cells, but having one good and one defective gene only sickles one-third of the cells, which hinders the development of any one of the four protozoan species responsible for malaria, so it comes out as an evolutionary advantage overall. However, due to modern medicine, malaria is treatable and a lesser concern.
- Similarly, it appears that having one copy of the cystic fibrosis defective gene grants some resistance to cholera.