Adaptive Ability

""That which does not kill me can only make me stronger.""

- Friedrich Nietzsche

This is an enemy whose power is to adapt to any power used against it.

Hit it with a fireball? Next time it's immune to fire. Same with any Elemental Rock-Paper-Scissors attack. Cut it, it now has armor. Psychic powers, it now has mindshields.

Logic would dictate the "easy" way to beat it is to use an overwhelming attack to begin with. However, this goes against most heroic codes, so by the time they've slowly cranked up the power of their attacks, the villain has "adapted" quite a bit.

Beings with an Adaptive Ability are very often Social Darwinists, if they're not mindless monsters.

The result is It Only Works Once. The good guys will easily be convinced the Evil Evolves. The functional opposite is Power Copying. A sub-trope of Feed It with Fire. Compare and contrast with Energy Absorption, where the defender is actually powered by being attacked. See also Adaptive Armor.

They have a high chance of being an Ultimate Lifeform.

Anime and Manga

 * In JoJo's Bizarre Adventure, the Big Bad from Part 2, Cars, uses the Red Jewel of Asia to instantly become the ultimate creature, with all of earth's evolution in hand and the ability to adapt to any attack. He was effectively immune to every possible attack because he could simply change into a form that resisted it before it even hit him. Even when thrown into a volcano he surrounded his body with bags of air to insulate him from the immense heat. He was defeated by being shot into space by the volcano to float around forever until he eventually ceased thinking, effectively making him dead.
 * Kuro Momotaro from Yu Yu Hakusho. His body possesses the ability to memorize the damage of any attack that hits him, while his weapons, the Steaming Spheres heal him from that damage, and give him the power to overcome it.
 * Probably a useless ability if he's killed in one shot though.
 * Many of the enemies in Saint Seiya are fond of declaring that they could not be hit by a technique once they have already seen it. Given our heroes' reliance on Stock Footage finishing moves, this seems like it would be a downright crippling handicap for them ...
 * The Saiyans from Dragon Ball can do this to a limited degree, but it usually isn't brought up.
 * The Saiyans are the embodiment of the Nietzche's quote. If they are near-death for whatever reason, when they are recovered, they will come back stronger. A few enemies were too powerful for this to matter much though.
 * The Dragon Ball Z RPG for the SNES had this implemented: If a Saiyan character won a battle with 5 or less HP remaining, they would gain a level immediately no matter what.
 * The Return of Cooler, anyone? The entire point in that sequel to Cooler's Revenge is that just as Cooler's body was being destroyed, his mind merged with a sentient super-computer/space-ship. The ship rebuilds him as a mechanical lifeform, and now, thanks to the ship's enormous resources, whenever Cooler's body is damaged, the ship simply repairs his body or fabricates a new one with whatever vulnerabilities it learned about removed.
 * This comes to a head when the computer decides Cooler's main flaw is that there's only one of him. Cue enormous army of Cooler robo-clones.
 * The D-Reaper of Digimon Tamers showed an impressive capability for evolution (then again, it operated like a quantum computer); Originally, it just killed by touching. Then it developed "agents", feet soldiers who kept damaging attacks away from it. These agents then got more and more specialised; some were gunners, others anti-air units, others were scouts, melee fighters, defense specialists, etc. Upon a debilitating EMP attack, it then integrated all of these capabilities into one massive being and overwhelmed the signal. Ultimately, it was considered to be invincible so the only way they could deal with it was to
 * In Magic Knight Rayearth, the monster Atalante can do this. (What happens when you decide that overwhelming force is the way to go... after you've let it adapt way too much already? Very bad things.)
 * One villain in one Tenchi Muyo! manga series had this power. Luckily for Tenchi, it turns out that Lighthawk Wings were much too powerful to copy.
 * Taken to an annoying degree with Naraku from Inuyasha. He rarely shows up, preferring to attack through subordinates, but when he does, he's always too powerful to take down with whatever attack was effective against him last time. One has to wonder why Sango and Miroku even bothered to come along with Inuyasha, as their attacks eventually became utterly useless against Naraku and they were pointlessly risking their lives.
 * The Vajra in Macross Frontier, in episode 17, survive a direct hit from a nuclear weapon because it had been used against them a few episodes earlier.
 * And yet Mecha sized K-bars Knives which have been used the entire series still work just fine on them... That said this isn't as insane as it sounds nor is the feat nearly as impressive as it sounds if one considers the actual physics involved. Read this and be enlightened. Executive Summary: Nukes aren't nearly as lethal in space and the range at which they'll do noticeable damage to anything even vaguely armored is surprisingly short. Nor were the hits in question really direct at least on those we see survive (and going by the stock footage used for the blast not all of them did...) as the missiles looked to be fired more for proximity detonation in the enemy formation then direct impacts.
 * Said range actually measures in tens or hundreds of kilometers even for the basic nukes (an elbow-pushing distance even at LEO), and reaction weaponry in Macross is, in fact, antimatter, that has much greater range. All this doesn't make much sense, but then, do the space battles as they are shown there really have any sense whatsoever?
 * No it doesn't range is related to yield as both are just radiation bursts. Actually anti-matter will waste allot of energy if used as a proximity weapon as many byproducts quickly degrade to non-lethal particles within a very short distance. It makes no difference though as footage for the later attacks shows that even after they adapted the weapons were still vaporizing them if they were reasonably close to the blast. That's not what I call immunity.
 * Back on the knife thing, don't forget that the knives are supported by the Valkyrie's point barrier system, which basically lets it cut through armor like butter. It's basically a Power Knife.
 * Byakuran in Katekyo Hitman Reborn used this to an extend. He just didn't wait until someone used a certain tactic against him, he simply communicated with himself in alternate dimensions and linked their minds together to make himself crazy prepared for anything.
 * Nurarihyon from Gantz had this as his power. In addition to adapting to become immune to attacks, he could also beat fighters at their own game by Bigger Stick adaptation.
 * from Bleach became something like this. Every time he became injured or close to death, the Hogyoku gave him a new form to counter it. At least until
 * In El-Hazard: The Magnificent World, Ifurita seems to become immune anytime she's hit with a magic attack. And then she uses it on you...

Card Games

 * In the Magic: The Gathering universe, the Slivers' more prominent traits were their fast evolution rate, adapting to overcome anything thrown at them. However, what made them so dangerous (both in the story and in game) was their species trait of sharing their adaptations with all other slivers in proximity.
 * However, in-game, the slivers' abilities were conferred onto Slivers on all sides, so a Sliver Mirror Match could turn out deadly. Not to mention that there are some slivers with negative effects thrown in just to counter all-Sliver decks.

Comic Book

 * Doomsday and (in the Silver Age) the Calculator and the Shaggy Man from The DCU.
 * Lots of anti-super robots in the Marvel Universe: later models of the mutant-hunting Sentinels (especially in the Bad Future of Days Of Future Past, no relation), the Fury (from Captain Britain's adventures), and the Shiva series.
 * The appropriately-named Darwin of the X-Men has this as his mutant power—he will immediately mutate whatever he needs to survive. Immerse him in water and he'll grow gills, burn him and he becomes fireproof, poison him and his body chemistry shifts to neutralize it, etc. It's really too bad that it only happens as an unconscious survival mechanism; with control over that power he'd be damn near omnipotent.
 * It should be noted that his powers don't always give him the ability needed to win a fight, only to survive. When faced with the Hulk, he gained the ability to teleport to the next state.
 * Random, another Marvel mutant, whose other power was turning his hands into guns.
 * There was a robot called appropriately the Super Adaptoid that did this. In one comic, it was defeated by Ben Grimm who was his normal human self at the time, but wearing a suit that still gave him Super Strength. Ben Grimm tricked the Adaptoid into copying his normal human self, then used the suit's power to punch the Adaptoid out in one blow.
 * An early X-Men comic during the Mimic's brief tenure as a hero pitted him against the Super Adaptoid, that had already perfected the abilities of the Avengers. After beating everyone else, including Professor Xavier, the Adaptoid fought the Mimic to a standstill until the Mimic threw the match and psychically convinced the Super Adaptoid to copy his powers. The Mimic's own power was to copy other people's powers, just like the Super-Adaptoid. The attempt to copy that power exceeded the Super-Adaptoid's capacity to duplicate. The feedback also temporarily shut down the Mimic's powers.
 * Apocalypse caused battles for evolutionary purposes. When the X-Men beat his men, he thanked them for weeding out the weak.
 * Lobo has this, as a side-effect of his Healing Factor. As with most of his powers, how effective or what it works on varies wildly between stories and writers. One notable problem arose when a magic spell de-aged him, and also resulted in him become immune to that kind of magic. Unfortunately, the traditional cure for his condition was from the same type of magic...
 * And Marvel's even-further-over-the-top Lobo parody Lunatik has been adapting for six hundred million years, having started as a microbe and eventually eating all life on his homeworld.
 * The Ultimate Hulk has this to a degree—he has adapted to environments as extreme as the (simulated) surface of Venus.
 * This is also the reason why his condition can't be cured—his body adapts to whatever agent is used to prevent the change.
 * Mainstream Hulk has this too, but to a lesser degree and with less consistency since it's one of his lesser known/used/liked powers. He's been shown adapting the ability to breath under water and survive the vacuum of space for extended periods.
 * Another X-Men mutant, Lifeguard, gets whatever power is needed to save someone else.
 * Marvel's alien Kree were stuck in an evolutionary dead end, unable to adapt or mutate; so after trying a number of less-than-ideal solutions (breeding hybrids with other species led to social stratification and prejudice, setting off a mutagenic bomb led to—well, a huge frikkin' bomb that killed half their population and left most of the rest sick and dying) they eventually turned themselves into the Ruul, who can self-adapt to any condition. Later writers seem to have undone most of this for no apparent reason.
 * One of Spider-Woman Jessica Drew's original powers was that she became immune to any poison after being affected by it once.
 * Legion of Super-Heroes member Nemesis Kid had the power to gain whatever abilities were needed to defeat any single opponent.
 * Whenever DC's Resurrection Man is killed by something, he is reborn immune to the way he died. In fact, when he was killed by extreme pain, he came back as a woman, because women are more resistant to pain.
 * Actually, they aren't.
 * Clearly, in the DC Universe, they are.

Fan Fic

 * Taken to its logical extreme and closing off all loopholes along the way: The Ranma 1/2 fanfic Relentless, where the Reikoku has an Adaptive Ability. It's an unusual take as in the story, the Reikoku adapts to the characters' techniques

Film

 * The enemy in the 2001 film Evolution and its Animated Adaptation, Alienators.
 * Syndrome's robot in The Incredibles was rebuilt between beatings. Hero A would do X to it and destroy it. Mark II is built without a weakness to X and beats Hero A. Hero B does Y to it and destroy it. Mark III is built without a weakness to Y, and so on. There is an Easter Egg on the DVD that shows all the different versions and who destroyed them. There's also a scene in the movie where Mr Incredible finds the computer showing which versions killed what heroes. It was also able learn to avoid tricks that worked against it before, but that really only made it as smart as the people it fought.
 * X-Men: First Class movie portrays Darwin's abilities in full. Ironically,

Literature

 * In the Sword of Truth series, is said to adapt to prevent one strategy from working on it more than once.
 * In the novel Meg, blinding the giant shark made it deadlier, as its other senses compensated for its blindness and now it could hunt during the day.
 * In Last and First Men, the Martians do this while fighting the Second Men. Over the course of several thousand years, naturally.
 * The shorter timescale is partly down to the Martians' very unusual biology and partly due to the fact that genetics was very poorly understood when the novel was written. The structure of DNA wouldn't be identified for another quarter century, for example.
 * In Greg Egan's Teranesia, an evolving organism is apparently able to anticipate future challenges and develop appropriately.
 * In Stephen King's The Stand, what made the superflu different from the normal flu (and much deadlier) is its ability to change to fight the antibodies the human organism generates to fight the virus, until your defenses can't stand anymore.
 * This trope is played for tragedy and to kill all remaining hope in the sci-fi novella With Folded Hands.... A man creates a perfect race of robots to cater to humanity and stop them from killing each other after his home planet was wiped out by civil war. It works too well and the robots begin stripping humanity of its freedom, planet by planet. A man uses rhodomagnetics, the force discovered by the creator and what allows the robots their highly advanced functions, to slip under their radar and attempts to assassinate the creator for dooming mankind. He's stopped, and the creator only then realizes what the robots are doing. He jumps from colony to colony, staying ahead of his creation while attempting to create a weapon that will destroy his lifeless homeworld, where the central network is located. He finishes and fires the weapon, only for it to fail. Robots arrive at his residence and inform him that after the assassin, they engineered a way to detect and neutralize rhodomagnetics applied against them, effectively shielding the planet from the weapon. Mankind's only hope to free itself from the robots is dashed by an Adaptive Ability.
 * The Vord in Codex Alera definitely qualify. When we first run into them, they're a bunch of creepy spiders who spin wax instead of webs. Then there are big warrior beetles, and little spiders that can take over your brain,. And that was back before they got scary. They could quickly create entire new species and generate massive numbers of them. They see human knights in armor? Suddenly there are heavily armored humanoids with swords to fight them. Make a wall of shields? Now there are enormous mantises with sickle-arms that can reach over the shields and disembowel you. Oh, you can fly with magic? How about warriors with dragonfly wings. Team up with huge wolf-men? That's okay, we've got a vord for that too. Build a massive wall just to keep us out? How about a Vord the size of a Gothic cathedral smashing into your wall! Wow, good thing you have that amazing magic that we can't quite seem to get hold of... oh. Nevermind.
 * And to make matters worse, they're obsessively xenophobic, want to Take Over the World, aren't fussed about whether they do it by assimilating us, slaughtering us, or sticking us in wax to digest to death, and are controlled by a Queen who's the stuff of nightmares all by herself.
 * The Inhibitors in Alastair Reynolds' Revelation Space Verse have this ability, there's some interesting character speculation about whether they are really adapting or whether they're downloading countermeasures from an extant database whenever they encounter a new weapon
 * The sword of Gryffindor in Harry Potter can't be destroyed; it simply takes in the power of whatever it encounters. For example, when in The Chamber of Secrets.

Live-Action TV

 * The Borg from Star Trek: The Next Generation are explicitly described as being able to adapt to anything used against them, so most weapons will only work two or three times before they're useless.
 * Note: that 'anything' doesn't include stabbing their drones in the face.
 * Also note that in Elite Force, Seven came up with the IMod (Infinity Modulation Weapon), a weapon with infinite, random energy signatures that change after every shot to take out the Borg.
 * The IMod only works because it unleashes devastating energy in a single burst (at least against the Borg), and also automatically changes energy signatures. Normal phasers and phase rifles can do the same thing as the IMod, but must be manually changed to new frequencies, and are limited in power. Human (and alien) imagination being what it is, it's not possible to keep coming up with random frequencies.
 * Except maybe by randomly punching keys.
 * An episode of The Outer Limits (the 90s series) had a man infested with Nanomachines programmed to heal and protect his body, which they did mindlessly and efficiently—he nearly drowns and grows gills, he gets beaten up and grows extra layers of bone, and his skin develops poison glands like a jellyfish, so no one can touch him...
 * Smallville's version of Doomsday, like his comic counterpart, has this ability; as his "mother" Faora put it, "What kills you makes you stronger".
 * The Replicators from Stargate SG-1 have essentially the same adaptability level as the Borg from Star Trek; they, too, cannot seem to adapt to being shot with a machine gun, until Season 5...Viva La Evolution!
 * In Mighty Morphin Power Rangers, the War Zord Cyclopsis acted like this after it was rebuilt by Lokar. In a bit of a subversion, that "evolution" disappeared after the Rangers retreated and when they came back to fight, they knew how to overcome it.
 * Also, Hatchasarus was repeatedly destroyed and would reassemble with more bling (though not necessarily directly related to what had killed him before.) He can't be taken down for good until Cardiatron (the giant, floating heart that is its central computer) is destroyed.

Multiple Media

 * Reidak from the Bionicle series.

Radio

 * Parodied by the Haggunenons in the radio series of The Hitchhiker's Guide to the Galaxy, who would shapeshift into seemingly random "adaptations" because they have "the most impatient chromosomes in the Galaxy".

Tabletop Games
""We fought them the first day, and our guns tore through them with ease. We fought them the second day and saw our missiles bounce of a thickened skin they seemed now to bear, so we turned our lances on them. We fought them the third day, and no cursed thing worked!""
 * The epic level D&D monster, the Infernal, has this as a specific power, only able to be affected by a spell once from the same spellcaster.
 * At lower level there is the skindancer, which has pretty good resistance to the last weapon type that struck it, and the last magical energy type that affected it.
 * In the red box edition, the master's level book described the Adapters, who develop a temporary immunity when struck by an elemental attack.
 * Warhammer 40,000 has the Tyranids:


 * Even worse: Not just one of them, but at least that entire fleet you are fighting can get that characteristic.
 * The Tau were able to exploit this: by getting their Kroot allies to take on Tyranids in forests, particularly agile but also rather fragile Gaunts were produced that had adapted extremely well to fighting arboreal warriors...then they drew the 'Nids out of the forests and opened up with their usual array of BFGs.
 * Mechanon is a standard recurring (and overarching) Big Bad from Champions. Each time he is defeated (even if destroyed, he's rebuilt), he returns immune to the last attack that killed him, and often with weapons to which his prior nemesis is particularly vulnerable. If fought long enough, he truly becomes unbeatable.
 * Unsurprisingly, Mutants and Masterminds features a power or two for this. "Adaptation" works like it says. Then there's "Nemesis"- every round you pick one person to be your nemesis, and you get X points worth of powers suited to fighting them (X being based on your power rank). The catch is that while you are free to suggest things, the GM has final say on what you get, and can distribute things however he likes (he could give you one strong useful power, 2 medium-strength, fairly useful powers, 10 weak powers of varying helpfulness...)
 * One Charm the Lunar Exalted have allows them to render themselves immune to the first weapon classification to hit them in a scene. This is somewhat mechanically awkward because weapons aren't classified in a way that makes this make sense as it's written, but it's the thought that counts.

Video Games

 * Pringer X, the ultimate bonus boss in the remake of Disgaea 2: Cursed Memories and Disgaea 4: A Promise Unforgotten possesses an ability that makes it completely immune to whatever special attack it was hit by for the rest of the battle. Made more complicated by the fact that when you fight more then one of them at the same time later on, all of them become immune to the attack.
 * Cubia from the first four .hack// PS2 games.
 * Most RPGs have at least one boss that adapts to the elements you attack them with, resulting in a literal Elemental Rock-Paper-Scissors as you try to switch the elements of your attacks to be the opposite of your last one or whatever it has shifted to. Can be frustrating if you misstep, because the wrong element will usually fully heal the boss.
 * The Tetramorph in Final Fantasy X.
 * At least one monster in every other Final Fantasy game as well.
 * Most Final Fantasy bosses are actually a variant: they're weak against one specific element and absorb all others, but can randomly change the element they're weak against. This is usually called "Wall Change" or "Element Change" or something similar.
 * Breath of Fire 3 has a lot of these as well.
 * Magus in Chrono Trigger; the Golem boss not only defends against the attack type used on it, but begins attacking using that type.
 * It's possible to use Golem's copy attack against it: by getting the right equipment and using the right attacks, it's very easy to force Golem to use attacks that are very inefficient against you. In a New Game+, it's stupidly easy to force Golem to use attacks that heal you.
 * Pokémon has two: the Porygon series and Kecleon.
 * More specifically, Porygon2's Conversion2 move and Kecleon's Color Change ability. Conversion2 is rather more limited, since the nature of Pokémon's Elemental Rock-Paper-Scissors means that the type it changes to is randomly chosen from the types that resist the last attack. Color Change is a more typical example.
 * Ironically, since Ghost and Dragon-types are weak against themselves, using such a move on Kecleon actually makes it weaker to these attacks then it was before (It starts off respectively immune and neutral to them). Oops.
 * Variations of the glitch MissingNo. had no type themselves, and therefore took the type of the last Pokemon loaded.
 * There's also an ability called Adaptability, but it only powers up move of the same type (more than usual).
 * The fallen Levant's (One-Winged Angel?) monster form in Jade Cocoon 2: he occasionally gains immunity to the element of the side of the BeastAmulet that's been used to attack him. When this happens, he summons a pair of lesser Divine Beasts of the same element he just immunized himself against and they prevent you from attacking him by becoming the only available targets. Attacks with any other element will damage him as normal, and attacks of the opposite element are particularly effective, but a few turns after the lesser Beasts have both been killed, the cycle repeats. So the fight continues until you've defeated him.
 * Warning Forever has this as its main mechanic: a time attack boss rush where the boss repeatedly comes back stronger in a way that seeks to counter how the player destroyed it in the last round. For example, if it was defeated by having its rear armor destroyed, its next form will have more armor on its rear. It also changes which weapons it adds depending on how effective they were (e.g. if it killed the player using lasers frequently in one round, expect it to have a lot more lasers in the next round).
 * Warlock Nether Protection and Death Knight Acclimation talents in World of Warcraft work somewhat like this. They have a chance to reduce the damage taken from the school of the last spell that hit you.
 * There's also some creatures, notably certain types of Voidwalkers, with an ability like this. Hit them long enough with one spell school and they not only gain an immunity for that type of damage, but can now use an AoE spell of that type.
 * Arcueid from Tsukihime claims such an ability. She was immune to conventional weapons to begin with, and gained immunities to conceptual weapons as they were used to fight her. When Shiki killed her, she was sure he used some obscure far eastern conceptual weapon. In a slight subversion, she can't become immune to that particular ability, either.
 * The manga adaptation gives the same ability to Roa, a vampire (accidentally) created by Arcueid. He demonstrates it in his battle against Ciel - by the end of the fight, her signature Black Key swords have become so ineffective that being impaled by several dozen at once barely phases him.
 * In the Nexus War spinoff Worlds In Global Battle Locked, Nexus Champions can do this with their aptly named Tattoo of Adaptation, something they didn't have in the original.
 * In Nethack, you. If you manage to kill a red dragon, eat his corps, and you're immune to fire. The next red dragon's fire breath might damage your equipment, but will not do much damage to you. Same goes for several immunities.
 * This is pretty much the point of the Zerg in StarCraft. Notably, their upgrade building is called the "Evolution Chamber," in which the Overmind pits hundreds of zerg warriors against each other and only keeps the strong, and is also the fluff explanation for the origin of Banelings.
 * Berserker from Fate Stay Night, Unless you are The Hero (or his Future Badass) or King Of Heroes, you will probably fail to kill him.
 * In Spore you edit your creature throughout the cell and creature stages to give them the highest chance of survival.

Web Comics

 * Oberon from the Web Comic Killroy and Tina would always come back with the properties of whatever "killed" him. The only way to permanently kill him is to throw him into a black hole (so he comes back as nothing). Unfortunately, he falls into a star instead. Just think, if the series hadn't been cruelly abandoned, something might have come of it.
 * There are the Arc Words "illumination goes boom".
 * Aylee from Sluggy Freelance (although her adaptations may involve spending months in a cocoon).

Web Original

 * SCP-682 of the SCP Foundation is a reptilian Omnicidal Maniac who is Made of Iron who also possesses this power. They try to shoot out it's eyes, it grows several new sets covered in armor plating. They try to poison it, it starts spraying out jets of high-pressure blood to get the poison out. Thankfully, the adaptations wear off after a short while.
 * This is the main reason they refuse to use nuclear weapons on it: if it survives, it'll be immune to pretty much anything.
 * 682 actually makes an interesting addition to this trope: it adapts to whatever is used against it, then harnesses it as a weapon. One of the reasons its so deadly is because the Foundation's failed attempts to kill it have given it so many new ways to kill people.
 * In the ''Global Guardians PBEM Universe, Mexican superhero Evolucion, is a Flying Brick with "reactive evolution" powers. He gains those defensive and sensory powers he needs depending on the situation he finds himself in (he gains night vision in darkness, he grows gills if he's underwater, fireproof skin if he's in a fire, and so on).
 * The Super Robot Omega, from the same setting, is programmed to redesign himself after ever defeat so he cannot be defeated by the same method twice.

Western Animation

 * The android Amazo in Justice League. Eventually gets so many powers he doesn't know what to do with them and decides to become a philosopher. Doomsday to a lesser extent as per his comic counterpart. Try to lobotomize him with heat vision? His skull thickens in time for the next fight.
 * Shane "Goose" Gooseman from Adventures of the Galaxy Rangers had this power combined with his Shapeshifting, due to being a genetically engineered Super Soldier with a Series Five Implant that also acted as a Restraining Bolt if need be.
 * In the 2007 Doctor Strange animated film, Strange is described as having the innate power to absorb and adapt a defense for any magic thrown at him. Thus he goes from on-the-skids Muggle to Sorcerer Supreme in the course of one film and a grand total of one Boss Battle.
 * In the third Fairly Oddparents/Jimmy Neutron Crossover, the two create their own supervillain to fight, but accidentally gives him this power because of a typo.

Real Life

 * Your own immune system adapts to infections. A vaccine is a weakened sample of a virus, your immune system uses it to learn how to combat the non-weakened version. As a result, you are unlikely to get any particular virus more than once. Chicken pox is a classic example, and since it's much more dangerous to adults, it's recommended to expose kids to reduce the chance that they'll get it after they grow up.
 * Inverted by the original antigenic sin where a prior mismatched infection makes the future related infection significantly worse.
 * Other parts of our body respond to damage by rebuilding the affected tissues and structures with greater strength. Repeatedly damaged skin forms calluses. Exercise causes muscle damage and stress, so the muscles rebuild with greater strength. Damaged bone will rebuild itself with greater density.
 * Some martial artists train themselves by striking at hard things. This will damage their bones, which rebuild. This leads to people who can smash bricks with their heads and have skeletons visibly different. Serious Muay Thai kickboxers kick trees.
 * Overdoing it tends to leave the practitioner with serious health problems later in life, as the body simply cannot cope with the stress. However, with no stress on bones and muscles at all (such as happens without gravity) leads to them turning brittle, weak and ultimately none serviceable. Like all things, best utilized in moderation.
 * Any population of organisms evolve. The word "evolution" is older than the concept of biological evolution. This is slow, noticeable changes to the environment usually take several generations at least.
 * Bacteria reproduce quickly, and evolve quickly as a result. Biologists study bacterial evolution in a lab because they don't have to wait too long for the results. Bacteria can also develop resistance to antibiotics quickly—which is why doctors tell you to finish your entire antibiotics prescription instead of stopping when you feel better. If you finish it all, there's a good chance you've wiped all the bacteria out. If you don't finish the prescription, the bacteria remaining may have evolved a degree of resistance and the next course won't be as effective.
 * So when you're taking antibiotics, think of the bacteria as your typical shonen hero, and antibiotics as your signature move. Wipe them out before they can learn to counter your signature move.
 * More to the point, don't stop to gloat when you've got them on the ropes—just kill them.