Happened in Bleach to several friends if Ichigo, most notably Chad and Orihime. It was initially thought to be an effect of his massive power, but was eventually explained as an effect of Hougioku hidden inside Rukia during the first arc.
Later chapters explained that there is a field or cloud that increases spirit awareness/power and migrates across the world. It just so happened to be on Ichigo's hometown.
Mahou Sensei Negima! does this a lot; any girl who keeps a close proximity to Negi will inevitably have either some sort of latent power, or gain a powerful artifact through a pactio.
It's not superpowers per se, but considering how Digimon Tamers started off with only three characters having Digimon, it'd be quite jarring for someone to skip ahead to the final saga and see close to TEN tamers, as slowly, over the course of the series, every major character gets a Digimon partner.
Justified example: anyone who becomes a Green Lantern, because all that it takes is wearing the ring. Recent stories have revised this, explaining that it takes a certain kind of pure-hearted willpower to command the rings. The cynical Green Arrow tried using a ring and could only fire off a single arrow of light before getting drained.
This is justified similarly with Iron Man and his supporting cast. Since the series revolves around suits of Power Armor, all it takes is Tony Stark lending the suit out for one reason or another, or simply designing a new suit for them to wear. Usually this happened when Stark was unavailable or he was trying to protect his Secret Identity. Over the years, Jim Rhodes and Pepper Potts have both worn Iron Man armor and had their own suits designed for them. Conversely, you have Obidiah Stane and his son who were business rivals that simply stole Stark's designs.
Spider-Man. Whoo. At first it's only Peter with superpowers. Then pretty much everyone he knows gets superpowers of their own. It varies if those powers turn them into allies or more villains.
In vanilla Spider-Man, we've got Harry Osborn and Miles Warren who were introduced as Muggles but later became superpowered.
This is played with in regards to Harry's father, Norman Osborn was introduced a few issues before it was revealed he was the Green Goblin all along. Spider-Man and the audience met the Green Goblin a few years before Norman Osborn hit the scene but Norman was introduced before it was revealed he was a super-powered villain. This decision supposedly led to Steve Ditko leaving the series since he felt it was unrealistic.
Mac Gargan was introduced as a private eye for one issue before turning into the Scorpion.
Ultimate Spider-Man is a bigger offender. In the beginning, Spidey is the only character in the main cast with superpowers. By the 100s, the Human Torch, Kitty Pryde, and Iceman are all hanging out at his school, Harry Osborne transformed into the Hobgoblin, Mary Jane got experimental goblin powers from an evil Spider-Man clone, and mutantphobic Liz Allen was revealed to be Ultimate Firestar, a mutant superhero Canon Immigrant. And after Gwen Stacy dies, she comes back to life as the Ultimate version of Carnage.
And then there's the recent Spider Island storyline, where all of New York City develops Spider-Man's powers, including long-time supporting cast members like Mary Jane Watson and J. Jonah Jameson.
In Supreme Power, a mutagen spread by Hyperion's falling spaceship gave powers to nearly every other superhero in the setting. All of them were children at the time; the Amphibian was "infected" as she was being born. Later, the military's attempts to duplicate Hyperion's abilities using this mutagen also gave way to Tom Thumb and Redstone.
Superdickery.com makes a running joke/drinking game out of this, telling viewers to "take a shot" every time Jimmy Olsen from Superman is shown getting some kind of superpower.
For most of the run of comic series Powers the two main detectives have both been muggles who investigated crimes associated with superheroes and supervillains. Although one used to be a superhero until he lost his powers. Things get turned around, however, when a contagious power acts as The Virus, and one of them gets infected with it. Meanwhile, the ex-superhero is getting a new set of powers as well.
This is what the parents of Tyler Marlocke in PS238 hope will happen by putting their non-superpowered son in an environment absolutely dripping with superpowers. In a subversion, it hasn't worked (yet, at least), though he is learning quite a bit from the city's resident Badass NormalCrimefighter With Cash, The Revenant.
Happens a lot to the nearest and dearest of Bruce Banner. Even if one discounts those who gained their powers from the same gamma blast that created the Hulk, there is still Betty Ross - briefly turned into the Harpy, Doc Samson - who uses the Hulk's own gamma energy to gain super strength, his cousin Jennifer Walters - who becomes She-Hulk due to a blood transfusion and Rick Jones - who time shared his body with Captain Marvel, was briefly a Hulk himself and is now A-bomb... a blue version of the Abomination... and Thunderbolt Ross and Betty are Red Hulk and Red She-Hulk, respectively.
Rick Jones wife Marlo is now the New Harpy, his old enemy Elliot "The Clown" Franklin is the Griffin, Brian Tablot (brother of Bruce's rival Glenn) is Grey, Gideon Wilson (the father of Bruce's friend Jim) is Mister Gideon... At this point it's as though everyone he knows somehow develops powers eventually, because you may find this to be his entire supporting cast plus interest.
An attempt at invoking this was rejected when Jim Wilson, Hulk's sidekick from The Seventies, was dying of AIDS and asked the Hulk (at the time with Banner's brain) for a transfusion to keep him alive, like he had done for his cousin Jen. Hulk refused.
Literally in Empowered; three of the members of the "Super-Homeys" alone got their powers from post-human STDs, and actually teamed up after meeting each other at a support group for it. One nameless superheroine contracted her powers from a cosmically enhanced cape, a guy who slept with a Cute Robot Girl found himself turned into a living robot himself, and one poor unlucky bastard named Protean (he used to go by the moniker "Glorpp") dissolved into a Blob Monster as a result of an STD he contracted from a Green-Skinned Space Babe.
The Fantastic Four rarely have supporting characters who don't have one power or another. Johnny Storm's girlfriends either turn out to be Skrulls or later become heralds of Galactus. Ben's girlfriend Sharon Ventura started out with powers but eventually turned into his Distaff Counterpart, becoming even more superhuman. They both dated Alicia Masters*
Sort of. Johnny actually dated a Skrull spy named Lyja who was impersonating Alicia, then dated Lyja again as herself.
who was mostly normal until her father, the Puppet Master used her for his own means. She also dated the Silver Surfer for a time and was briefly given cosmic powers. Then we come to Reed and Sue. Their kids have godlike powers, their nanny was a sorceress who trained the Scarlet Witch, and Reed's dad was eventually revealed to be a time traveler. Even Doctor Doom is not exempt from this. His adopted heir Kristoff ended up in a suit of Power Armor similar to Doom. Wyatt Wingfoot is a friend of the family and the only one without powers, it seems. Despite that, he is still a Badass Normal who has gone toe-to-toe with supervillains.
As mentioned in the Western Animation and Comics sections, Spider-Man has a knack for this. Even the movie version. Both Osborns were introduced before gaining powers as the first and second Green Goblin, Peter met Otto Octavius a few times before he turned into Doctor Octopus, and he knew Eddie Brock before he turned into Venom. This is actually against canon since part of Venom's initial concept was that he was an unknown reporter who had a very personal vendetta against Peter. The idea being that Peter's heroics destroyed a life without him realizing it. The only villain Peter faced who he didn't know previously was the Sandman, although he was Uncle Ben's killer so there was at least some personal connection to him.
In The Amazing Spider-Man, Peter plays a role in Dr. Conners' transformation into the Lizard.
In My Super Ex-Girlfriend, a radioactive meteorite gave G-Girl her powers. When Hannah touches the same meteorite, she gains the same powers as well.
In Reflex, the sequel to Steven Gould's Jumper, Millie gains Davy's teleportation abilities by being teleported by him a large number of times.
In Tik-Tok of Oz, Dorothy finds out that Toto can talk, just like all the other animals in the Land of Oz.
In the Discworld, magic is very much like this. Leaked magic from Unseen University causes strange behavior in ants, cockroaches and a dog to become able to talk and understand at a human level (much to said dog's annoyance).
The pets of wizards in the Young Wizards series tend to "become strange", with the most common powers being increased intelligence and the ability to speak. Tom and Carl have a dog with super strength and a macaw that can look into the future, while Kit's dog Ponch develops the ability to create new universes.
After Kit uses magic to fix a remote control that doesn't work properly with its TV, the TV starts spontaneously developing new features, like receiving alien cable stations and hooking into alien chatrooms.
In the Darkover novels by Marion Zimmer Bradley, the Ardais family's laran- their hereditary psychic talent- is catalyst telepathy, the ability to awaken laran in others, often simply by living in close proximity to or interacting with their targets for long periods of time.
In the Wild Cards books, powers are created by the Wild Card virus, which is not normally contagious. However, in one story, a character develops a form of the virus that can and does infect anyone he comes in contact with.
Played with in that the virus is contagious, but the powers might not be. An ace passes along the virus genetically, but the kid still draws the same odds as every one else of dying, becoming a joker or becoming an ace.
In the last TV episode, Buffy and Willow give hundreds of girls the same powers as Buffy, permanently. In the Season 8 comics, Buffy is still the main character, not because she's the most powerful, but because she's the leader of the good guys.
The beginning of Heroes had several Muggle characters. Then they were slowly killed off, Put on a Bus, or caught Contagious Powers of their own. By now the only major cast members without superpowers are Mohinder, Ando, and Noah Bennett's family sans Claire. And really, it's probably only a matter of time for at least one of them as well.
Some fans speculate that Mohinder's immunity to the Shanti Virus may qualify him for Hero status, though that's iffy considering he's only immune to one strain of the virus. Then again, blood type incompatibilities with his transfusion to Molly, The Haitian, and Niki haven't come up.
The lack of blood type incompatibilities could just mean he's Type O Rh negative, universal donor (as is the case of 2% of the population of India). Or just Type O in general (about 38% of the population of India) considering that 90% or so of the world's population is Rh positive and thus compatible. . .
The first season of the 1990s The Tomorrow People series, Megabyte is the only one of the characters without powers, but then (surprise!) he gets them in the last episode of the first season.
Smallville. Thanks to their dominance in the Green Rocks market, the citizens of Smallville (often kids at Clark's school that he "knew") receive powers quite frequently in the early seasons, and of course Clark's supporting cast gets sucked in eventually.
Lana gains precognitive visions for one episode. Several seasons later thanks to lightning she gets Clark'sFlyingSpeeding Brick powers. Naturally, she loses them by the end of the episode but then later gets brick superpowers before leaving. Oh and she got vampire powers then gained heat vision by drinking Clark's blood some time in between.
Chloe was briefly able to make people tell the truth, before the producers decided to give her a set of Healing Powers. Then her already improbable ability to hack anything was upgraded to superpowered proportions... which vanished when Brainiac is removed from her body... and the healing power has seemingly been forgotten.
Lex, it was hinted had an advanced immune system. He also received a set of fancy Kryptonian powers through an invisible power suit.
Pete briefly gained Stretch powers for an episode thanks to Stride Gum.
Jonathan is also in brief possession of Kryptonian powers for an episode, although it's suggested the strain it put on his body significantly shortened his life expectancy. He also somehow healed Clark once although that may or may not be Jor-El working.
Oddly enough, Clark, who already had Speeding Brick powers and getting new ones each season, was gifted with "Dead Zone" Visions for a single episode in the middle of Season 2 although it might be the artifacts rather than his innate ability. In season 9 he even gained two one-episode powers.
And of course - Lana, Chloe and Lois ALL get superpowers when they're possessed by witches.
In Lois and Clark, twice Superman's powers were transferred via lightning. One was permanent (until removed deliberately), and the other temporary. The villain of the week also attempts to copy the superpower-copying lightning in a lab enormously. She succeeds, turning a mouse into a superpowerful killer of cats. She briefly gains Kryptonian powers until beaten and the device used to reverse the effect. Strangely, this does not happen more.
In one episode, Lois gets Clark's powers and spends the episode as Ultra Woman.
The 4400 took this trope Up to Eleven. It started with 4400 returned alien abductees, a few of which had superpowers. Then the government stops giving them superpower suppressant, and it turns out they ALL have superpowers. Then a scientist isolates the magical neurotransmitter behind it all, enabling him to synthesize a serum that can give ANYONE superpowers (though with a 50% risk of death). Then one guy, taking the serum, gains the power giving anyone around him superpowers (again, with a 50% risk of death for each person).
At the start of Persona4, only the protagonist has the power to enter the TV world at will and summon his Persona. Through the course of the game, everyone who joins the investigation team picks up both of these abilities.
Not quite a purely accidental example, but: In the Whateley Universe, Generator picked up her roommate Tennyo's regeneration power in a mad science-powered attempt to get her seemingly permanent 11-year-old body 'unstuck' by using Tennyo as a template donor. Of course, there's also the bio-devisor student Jobe Wilkins who, if he put his mind to it, would be able to come up with quite a few ways to create literally contagious superpowers if his lab accident with the 'drow formula' originally meant for a prospective girlfriend is any indication...
And then there's the way Residue got her powers.
The third season of Static Shock starts out with Richie getting super-intelligence powers and becoming the technologic superhero Gear. Virgil hypothesizes that Richie was exposed to the trace amounts of the mutagenic gas left on his clothes the night of the Big Bang, hence why Richie's powers took over two years to manifest. This makes it a literal case of contagious powers.
Ben 10: Season 3 Gwen starts training herself in magic, after refusing the call the previous season.
In the 90s Fantastic Four cartoon, Johnny was afraid this had occurred when he kissed this foxy redhead and she immediately burst into flame. Turned out she had some kind of mutant power she'd repressed due to accidentally burning her house down as a child.. Then played straight when she got extra powers from Galactus in order to become his herald.
The Spectacular Spider Man does the same, introducing the pre-Goblin Osborns and pre-Lizard Dr. Connors, making pre-Venom Eddie Brock a regular, and introducing pre-Sandman and Rhino versions of Marko and O'Hirn as recurring petty thugs before being supervillain-ized. We also met John Jameson and Mark Allan before they became Colonel Jupiter and Molten Man, respectively. Black Cat appeared in-costume first, though.