Goku, the protagonist, would not have been the hero he became had he not been raised by Gohan Sr. Of course, getting Identity Amnesia as a baby and forgetting his purpose was to wipe out the human race probably helped!
Even Vegeta the callousalienprince who wiped out hundreds of worlds and cared only about becoming the strongest in the universe eventually softened after he permanently moved to Earth, most notably settling down with a human wife and raising their half-Saiyan children.
And then of course there is Piccolo, the former Big Bad who was the solidified personification of God's evil and spite, who once wanted to take over the world...
"Pathetic... to think that Demon King Piccolo would fall... protecting a child... I have been.... contaminated... by you... and your father's decency... Gohan."
Slightly more subtle example: Celty from Durarara!! wonders if her time living among humans in Japan may have caused her to adopt some human values, most notably that over time she has come to think of Shinra as more than just a roommate. Of course, we don't know exactly how "inhuman" Dullahans normally are and it seems that Celty was pretty kind in the first place, so even she can't be sure exactly how much it's affected her. Celty herself seems to have just decided to go with the flow and let it happen.
In BleachUlquiorra before he dies, begins to understand, and maybe even become similar to humans.
This is basically the character arc of Deedlit in Record of Lodoss War, especially in the manga, alongside Curiosity Causes Conversion. She becomes fascinated with why humans are the way they are, and eventually, she's even accused by her fellow High Elves as becoming corrupted by human lines of thought.
The Zentradi race in Robotech suffer this after they send infiltrators onto the SDF-1. Being pure Proud Warrior Race Guys, the concept of civilian life, love, music and peace was alien and alluring. In fact, it lead to an Enemy Civil War.
In probably one of the largest examples of this trope ever, the introduction of these human concepts led to the defection of over one million starships and countless billions of Zentraedi to the human side. However, the Zentraedi loyalists still have about five times that many ships, so it's not quite the ideal situation...
Humanity Is Infectious is one of the strongest weapons humanity has in the Robotech series, and in the parent series Macross. In pretty much each arc, our culture is not only used as a weapon, it allows us to make friends with beings who have Blue and Orange Morality to a terrifying degree.
In a recent Mahou Sensei Negima! chapter, we see that this has happened to the third Fate Averruncus. It does help that he wasn't created to have a one-track mind like the ones before him.
Ronnie Schiatto of Baccano! got entirely bored of being an omnipotent, omniscient Eldritch Abomination after the Advena Avice incident in 1711, so when Elmer asked him to stick by Maiza as a human until the alchemist could learn to smile again, he took the opportunity. Maiza somehow wound up in the Camorra some two centuries later, and Ronnie learns how damn good it feels to be a gangster.
Maiza: Ours is a demanding business, is it not.
Ronnie: Do you wish you were still an alchemist?"
Maiza:...No. No, I don't. There was a time when I regretted ever summoning you on that boat... but I do not for a second regret my place here right now.
Maiza: Hah... The same goes for me.
In a Pre CrisisFlash story, the intelligent apes of Gorilla City revealed their existence to humanity, only to regret it after the concept of "leisure time" was introduced into their society, turning many of them into couch potatoes. Their solution? Brainwashing the entire human race into forgetting they existed, so they could go back to hiding! (this was a pretty ham-fisted way to retcon them back to their original Status Quo.)
An X-Men story had a nurse infected with an parasite from a race of evil aliens (obviously a Captain Ersatz for an Alien Queen) but her Christian faith and love for people were so strong that neither she nor the people she'd infected fell to The Corruption.
The third Blue Beetle, Jaime Reyes, has formed a sort of symbiotic bond with a piece of alien AI that was meant to take complete control of him. By the time he's done with it he's taught it free will and heroism, among other things, and there are hints that these things have been spread throughout the entire alien infiltration program.
The Reach commander Big Bad stubbornly refuses to believe that Jaime's simple heroism and good nature changed the Scarab and wastes valuable time and resources trying to find a more concrete explanation. His Dragon is able to believe it, and criticizes his boss for being in denial.
All the aliens and powerful beings who have tried to "fix" Superman's silly humanistic tendencies have found that humanity is really, really hard to cure.
Viltrumites seem to be particularly susceptible to some kind of infectious humanity. Nolan Grayson/Omni-Man, the first envoy the viltrumites sent to Earth to prepare it for annexation, quickly learned during the course of his heroics that while humanity can collectively be very brutal and monstrous, outstanding individuals can be endearing and amicable. He came to love one particular human woman and married her, and found that he could not deny his feelings for his wife even when he tried to suppress them in accordance with viltrumite tradition.
When the remaining viltrumites immigrated to Earth to breed and replenish their severely depleted ranks (because human and viltrumite genetic code is so similar that the offspring of a human and a viltrumite has complete viltrumite powers), Grand Regent Thragg expected his people to maintain their imperious culture and not form attachments to the humans the viltrumites took as mates. As he remained isolated on the Moon, Thragg noticed that more and more of the viltrumites who mingled with humans began to accept human culture and behave in ways that showed attachment and care for their unwitting human partners and lovers. In issue #101 Thragg gives a dressing down to three viltrumites who explicitly go against his repopulation plans; two of them, General Kregg and an as-yet unnamed male viltrumite, explicitly express affection for their human mate or mates, explaining "To have someone care for you…to be able to think about them…the bond that forms [is] amazing. It changes everything, Thragg." (The third dissident viltrumite, Anissa, both never wanted children in the first place and also thought the humans she had encountered to be inferior and dissatisfactory, so it's no wonder she would refuse to have a child with one of them.)
In the Buffy the Vampire Slayer fanfic Difficult to Fight Against Anger, after Anya confesses to Xander that she nearly did the vengeance thing on him, but couldn't bring herself to do it, Xander says humanity's rubbing off on her, to which her response is, "Yeah. Kind of sickening."
In Kyon: Big Damn Hero, this is the reason for the Integrated Sentient Data Entity's planned deletion of Yuki.
Films — Animated
The Last Unicorn uses this trope to horrifying effect by showing exactly what happens when you put the mind/soul of a pure, immortal unicorn in a base mortal human body.
Amalthea: I can feel this body dying all around me!
Used again in a less horrifying and more tragic fashion when she falls in love for the first time. She almost considers giving up on her quest and marrying the guy. He convinces her that giving up isn't an option and that the quest must be seen through to the end.
Played tragically at the end. Despite rescuing the other Unicorns and returning to her true form, the ending is given a bittersweet twist as it's revealed her time as a human has left her with the unique (among Unicorns) ability to feel regret. Being the ageless kind of immortal, that means she'll have to live with the pain of her regrets and losses for a very very long time.
Films — Live-Action
The T-800 in Terminator 2: Judgment Day. Originally coldly robotic, its neural net processor ("learning computer") picks up human slang and attitudes from the Connors.
T-800: I know now why you cry. But it's something I can never do.
Even the T-1000 picks up a few mannerisms. At first, it's only using a personality in the process of better infiltrating humanity in order to kill John Connor. By the end of the movie, it likes to silently mock the protagonists' futility via Finger Wag and taking its sweet time to attack Sarah Connor for no other reason than For the Evulz.
Note that the T-800's chip had its ability to learn from its surroundings turned on by the Connors. In the director's cut, at least.
Starman doesn't quite make it all the way to human. His gait and mannerisms remain stiff and quirky, but emotionally, he gets it.
In Teenagers From Outer Space, reading a single human book is enough to convince the alien Derek to turn against his space-Nazi brethren and side with humanity.
Daybreakers does this literally: the best cure for vampirism is the blood of an ex-vampire.
This is the entire premise of the film The Nines, wherein the protagonist turns out to be a demigod who created the local universe and the humans in them, fell in love with his creation, and has been slumming it among them ever since to the point of even forgetting that he is a god.
Avatar is an inversion: Jake finds that Na'vi-ness is infectious, and turns against his fellow humans to save the native population. The movie follows the structure of alien-invasion plots, but with the roles of human and alien reversed.
Robocop is about a police officer who loses his humanity by becoming a cyborg only to later gain it back.
In the book Thief of Time, Myria LeJean, Auditor of Reality, has been a person for so long that she got a personality and joined the Good Side fighting her former "comrades". Sadly, she commits suicide because she believes she has no place in the world she helped save.
Her former comrades are also affected, it's just that in trying to fight it they end up with such human charactersitics as anger, argumentitiveness, and homicidal insanity, rather than free-thinking and compassion.
In the co-authored book Good Omens, this happens to the angel Aziraphale and the demon Crowley as well: Aziraphale becomes a little more lax about rules and when to follow them, while Crowly develops a bit of a conscience and takes a liking to humanity.
Mort shows that the inverse is also true for Death- As Death neglects The Duty more and more, his assistant Mort begins to gradually gain Death's powers, mannerisms, personality, and even his voice. Apparently, Death is whoever does Death's job.
Men at Arms describes dogs as being wolves that have been "infected" with human traits, the good and the bad.
Throughout the series, many creatures are described as "almost human, really", such as a Troll who lends at 300% interest per month, or Dragons fighting to death rather than submission.
In Proven Guilty, the Winter Lady Maeve expresses concern that her late counterpart in Summer, Aurora, fell victim to human concepts like "hearts, good, evil."
In Liz Williams' Inspector Chen series, one of Chen's friends is a demon who's developed a conscience from being around people too much. In the scene where this is first mentioned, he describes it as if it were literally something infectious he'd caught off humanity, and claims that the only reason he hasn't had it seen to is that as a minor public servant his health insurance doesn't cover it.
Literal (and quite unpleasant) variant in Isaac Asimov's short story "Hostess". It appears that ageing and "natural" death are results of a parasitic life form, which infects all humans. Aliens don't age, and their death is either accidental or voluntarily. Except that now the parasites start infecting aliens, too...
A nastier-than-usual version in the Star WarsExpanded Universe: when the Killiks inadvertently assimilate Raynor Thul into their Hive Mind, they begin to value individual life—even though individual Killiks aren't particularly sapient and reproduce extremely fast. Next thing you know, they're trying to expand their territory like crazy, nearly precipitating a war with the Chiss.
In Animorphs novel Visser we learn that Visser One very nearly fell into this trap when she discovered Earth, and she had a partner, Essam, who fell into it completely. In varying degrees, there are any number of Yeerks in the main series who start acting and identifying more "human" than the Yeerk Empire would want them to.
MorningLightMountain over the course of Pandora's Star' and Judas Unchained as an accident of uploading Dudley Bose's personality into its brain to learn about humans. Turns out this is a bad thing because MorningLightMountain's inherent urge to expand get's augmented with things like hatred'' for all things not MorningLightMountain.
This is a significant theme of Robert A. Heinlein's novels dealing with intelligent computers. It occurs first in The Moon Is a Harsh Mistress, when the computer Mycroft Holmes learns to be human (and enjoy it) through observation of those around him. Similarly, in Time Enough For Love, Lazarus Long expresses the opinion that the thing that turns a computer from merely a powerful machine into a fully sapient AI is The Power of Love — specifically, being loved and paid attention to by a human. This can then develop into full-blown Pinocchio Syndrome.
Averted in C.J. Cherryh's Foreigner series. The alien atevi experience no emotions which are forms of affection, and they never will, because which emotions an organism can experience are determined by neurological hard-wiring, and atevi don't have the hard-wiring for those emotions. Similarly, the atevi do have the alien emotionmanchi, an emotion that humans will never be able to experience, no matter how much exposure they have to the atevi.
The World War series by Harry Turtledove has cultural infectiousness going both ways. While we adopt lots of The Race's technology, and their practice of not wearing clothes and instead wearing body paint, and even their practice of calling things "hot," (similar to slang for "Cool,") we managed to introduce the Race to The Oldest Profession, drugs, and marriage. It's more difficult to see the contamination of the Race's culture, though, because of how slowly their society moves.
Right up until Homeward Bound, where we see members of The Race wearing wigs, which comes as something of a shock to both the human protagonists and their 'mission to earth' opposite numbers.
In the Starfleet Corps of Engineers stories, P8 Blue, an insectoid alien, finds that her belief systems are influenced by the humans she works with. She begins to find an interest in history where her people usually ignore it, and even feels slightly maternal towards her larvae, being a little sad when she drops them off at the childcare centre, never to see them again.
Deliberately averted in Isaac Asimov's Robot stories. Asimov disliked what he called "robot-as-pathos" stories, preferring to see robots as highly complex and intelligent tools. However, Asimov's novella The Bicentennial Man and the movie based on it fit this trope.
In Ray Bradbury's The Martian Chronicles, the Martians (who have psychic abilities of some kind) gradually become infected with human memories to the extent that their entire culture goes insane and is pushed to the point of extinction.
In Leviathan Wakes, Miller is able to negotiate with Eros because the protomolecule which infected Julie Mao used her personality as its foundation.
In David Brin's The Uplift War, one alien is studying a human language with derision. Take that messy term "accident" — why the humans even had a saying, "There are no accidents." At the end of the work, while reflecting on what resulted in their defeat, and how things could have gone differently, the alien realizes that nonetheless "there are no accidents."
Legacy Of The Draconica: Spending ten years in a human's mind absorbing emotions and watching him interact has allowed a mindless weapon (Mordack) to develop a sense-of-Self.
In the Doctor Who episode "Dalek", this happened to the Dalek upon exposure to Rose with a dash of Humanity Ensues, since the Dalek started growing emotions as a result of having physically assimilated Rose's DNA, as opposed to just of hanging out with humans for awhile. It freaks out and self-destructs.
It has been theorised that this trope has happened with the Doctor to some extent, although given that his personality changes with each regeneration it's a bit hard to pin down exactly how much humanity has rubbed off on him.
Note that the above Dalek would literally rather die than be anything like a human being, and self-destructed.
Inverted when Rose's mother Jackie worried that Time Lordiness is infectious in Army of Ghosts, when she suggested that after she'd died that Rose would never return to her home time or planet and continue to travel with the Doctor forever having completely lost her humanity in the process.
This has actually happened to Daleks a couple of times. The first was the serial The Evil of the Daleks, in which they try to isolate the "human factor" that allows humanity to continually resist and defeat the Daleks. Those Daleks that are exposed, however, perform a Heel Face Turn and a civil war errupts.
More recently, the Cult of Skaro tried to do something similar and hybridize themselves with humans to discover why we are such great survivors, while their race is on the brink of extinction. The first Dalek to do so, Dalek Sec, also performs a Heel Face Turn, and is exterminated by his brethren.
Spock: At the moment, that is all we can do, except hope for a rational explanation.
McCoy: Hope? I always thought that was a human failing, Mr Spock.
Spock: True, Doctor. Constant exposure does result in a certain degree of contamination.
There's a lot of this in Star Trek. After the normally selfish "omnipotent" being Q from Next Gen spends some time as a human, he ends up sacrificing himself to protect the crew - and is given back his powers because of it. He then becomes more of a Trickster Mentor to Picard then an outright antagonist.
The whole premise of V is that the aliens, when living among the humans long enough, become disillusioned from their leader Anna and rebel.
Stargate Atlantis: Todd the Wraith became considerably more human during his imprisonment by the Genii.
Illyria from Angel has this as her main storyline, including the fact that she seems to feel that the ways of humanity are literally an infection.
The longer Castiel hangs out with the Winchesters of Supernatural, the more human-like he becomes. Recently he was watching porn, which is especially odd considering he's an angel.
Delenn in Babylon 5 gets some of this after Literally becoming part human at the end of the first season/beginning of the second. She's eventually kicked off the Gray Council for exactly these reasons, even though there hasn't been a demonstrable change in her behavior at that point.
This is actually part of the reason that Humans Are Special in Babylon 5: Humans are empathetic, and therefore weave disparate groups into communities, with a common purpose. Babylon 5—a place where many species gather to live and do business with each other in (relative) peace and harmony—to say nothing of discuss their differences in a permanent, neutral forum—would have been literally unthinkable to another species. The smartest aliens are the ones who get this concept and sign on to it as much as circumstances allow: G'Kar for the Narn (who writes the Constitution of the Interstellar Alliance) is the greatest example, but Vir and even Londo, for the Centauri, come to accept it to a degree.
Stargate SG-1 has a great deal of this, most evidently seen with Teal'c. He eventually gets an apartment, trades in his Goa'uld-issued Staff Weapon for duel-wielded P-90s, and adores Star Wars to a somewhat unhealthy degree. When asked to come up with an example of virgin birth, the audience thinks of Jesus. His immediate answer? "Darth Vader."
John Crichton has this effect on most aliens who come into contact with him. The change is most pronounced in Defrosting Ice Queen Aeryn who notes after Talyn!Crichton's death, "He made me better."
An interesting example lies in the True Fae from Changeling The Lost. One of their defining elements is that they cannot understand empathy; if they adopt emotions, it's merely the mask of an emotion, not something deep seated. Any True Fae that actually learns how to think like a human becomes a Charlatan, losing all memories of their existence in Arcadia and a good chunk of their powers as well. Problem is, such a state can easily be reversed...
Any emotion works, incidentally. Love does the trick, but one sample character became a Charlatan on connecting with the madness of a Serial Killer, and one spent so much time stalking his brother's killer with a knife, fueled by hatred for him, that he became a Charlatan and forgot why he wanted him dead. All he remembered was that the man did something terrible to him, so he just keeps stalking.
A similar effect with the Alchemical Exalted, in that they typically start out with a relatively human outlook but become steadily more icy, ruthlessly efficient, and generally computer-like as their Clarity rises, until they become closer to machines than men. What's one of the easiest ways to bring Clarity down again? Simple. You interact with humans.
A possible example occurs in World of Warcraft in Ulduar, where the raid group beats the tar out of Algalon the Observer and, in doing so, effectively cause him to feel empathy for the first time. He enters a recall order to a prior order he'd sent to have Azeroth scrapped by the Titans.
The Big Bad of Persona 3 grew empathetic to humanity through his time residing in the consciousness of the protagonist and, though he couldn't stopThe End of the World as We Know It, offered SEES the chance to live out their lives in ignorance of it, so they wouldn't be afraid.
Also Aigis, who develops human emotions during her time with SEES and through her Social Link plotline with the protagonist.
The sequel has another dark take on this trope: a spirit representing a virtue (Justice) can become twisted into a demon (of Vengeance) through exposure to the emotions of the human it's possessing (namely, rage at the injustices of how mages are treated).
The Materials of the Magical Girl Lyrical Nanoha As Portable games are Humanoid Abomination created from the remnants of the Darkness of the Book of Darkness. They were supposed to be Omnicidal Maniacs like the Darkness of the Book Darkness was, and in the first game, they were. However, thanks to their interactions with the characters in said first game and their own budding sentience, they returned in the sequel a lot more human than before. Not only did they gain their own Dark Pieces, which are only formed when someone has their own hopes, wishes, and regrets, but they've also become individual enough for two of them to reject their original Omnicidal Maniac goal as something that is both obsolete and runs contrary to their new goals in life (i.e., making friendly rivals out of their former enemies).
The Puyo Puyo series has Ekoro, a Spacetime Traveller. In his first appearance, he only sees humans as silly and as something that gets in his way of crushing all existence in Puyo. By the time 20th Anniversary rolls around, thanks to a case of amnesia, he starts to learn about how to act and what is and isn't considered "fun". The hidden unlockable costume for him shows that he has a case of Humanity Ensues.
Ahri from League of Legends is a kumiho (the Always Chaotic Evil Korean counterpart of the Kitsune) who was convinced that she could become a human if she killed and ate the souls of enough human beings. It turned out to work a little too well — as she became more human, she began developing human traits that she didn't previously have... including a conscience. She eventually swore off murder and joined the League to find out if there's a way to complete her transformation without any more killing.
The reason given as why EDI in Mass Effect 3 eventually comes to self-identify as being human, due to the influence of the two most important people in her life, Shepard and Joker. You can see the beginnings of it in in Mass Effect 2 where as EDI learns from Joker, it starts to absorb more of his personality including playing practical jokes.
Happens with the Geth known as Legion, who is utterly perplexed by the fact that of all the available material at hand, he specifically used a piece of Shepard's armour to patch himself up... because he's become a fan.
Likewise, compare Legion in Mass Effect 2 to how it is in Mass Effect 3, or to the unnamed Geth AI that replaces it if either destroyed or never activated during 2, which is a backup copy of Legion's platform made before it left the Perseus Veil in search of Shepard. It's abundantly clear that the time spent with Shepard's crew changed it a lot.
Star Ocean The Last Hope has Arumat. In the game, he's a textbook example of Good Is Not Nice. In his bonus ending, he is visibly much more outwardly heroic in nature, and he comments on it himself. He blames being exposed to humans, especially Edge and Reimi, for this character change.
In Homestuck, the Trolls' first contact with humanity is conducted by sending death threats and harassing messages to the protagonists over the internet, but as both groups realize they have to work together against a common foe, several Trolls quickly and unconsciously begin adopting human mannerisms and figures of speech, even forming romantic obsessions with the kids.
One forum post on 4Chan was about how an alien race uses Memetic weapons, that is, weapons of ideas. It's considered a very serious weapon of war. They are outright horrified when they hear that not only is it legal on earth, but it's considered a game: whoever can spread ideas, or "Memes" the furthest wins. That's right, humans managed to weaponize their infectiousness. And they did it by accident.
Happens in The Salvation War, (chronologically) first with Michael (yes, the archangel), and later with the demons, once we beat them up hard enough that they're ready to sit there and pay attention.
Robot: I'm afraid I can't. I'm starting to actually feel what you humans call compassion. It's an amazing feeling.
Domestication is essentially applying this to animals.
It's amazing the range of pets that show more self-awareness than non-pet members of their species. There are even reports of a pet crustacean showing playfulness uncharacteristic of its species.
This is not, however, the only mental difference between dogs and undomesticated wolves. Dogs are wolves that have been selectively bred to be a Slave Race.
Research suggests though that 'friendly to humans' behaviour is actually genetic, rather than septic.
Dogs do not naturally give or receive social cues with their eyes, relying instead on body language and vocalizations. Domesticated dogs have learned to watch the eyes of humans for social cues, and might even be learning to give eye-based social cues themselves.
Reports of children who've been Raised by Wolves suggest that this even applies to humans – being members of the species ''Homo sapiens" helps, but much of what makes us human is being raised by them.
But being infectious appears to be an inborn instinct, apparently evolved for childrearing but also incidentally applied to pets.