Harry Potter. Wreck-It Ralph. The Office. Cowboy Bebop. What could a fictional video game character and a boy wizard have in common with a paper company and cowboys in space? They have fans. They have fanatics who absorb every scrap of material available and commit it to permanent memory. They have people who buy up all the merchandise and catalog it like a museum’s archives. Fans are proud of the little things they call their own.

A fandom is a social community built when people who enjoy a certain type of entertainment (whether it’s a TV show, sports team, or movie) get together and share ideas about it. People often meet friends through fandoms, or buy, sell, and exchange fan-made merchandise. My sister, a die-hard Broadway geek, has a thriving Playbill trade: she sends some of her own collection of 100-plus across the country in exchange for Playbills from different shows. Some fans engage in cosplay, a type of role-play where people dress like their favorite characters in either store-bought or homemade costumes. These hobbyists work for weeks and sometimes months to perfect an ensemble that they then wear to fan conventions.

So what’s it like to go all-out nerd for something? Is it different for every fan, or is it a universal experience? What attracts superfans?

“Just the fantasy,” says Katie Reed, a proud Whovian (a fan of the space- and time-travel TV show “Doctor Who”).“Travel and save the world? I wanna do that!”

“And the creativity,” says Olivia Austin, who cleaves to no single fandom and instead has tried several on for size. “The variety’s good.”

Here’s how it starts. You see the cover of a book or the description of a TV show, and you think it might be worth your time. That’s what I like to call “the beginning of the end.” Once you start absorbing the world of your own personal phenomenon (and yes, you get to a point when you think it belongs to you), you don’t stop. Maybe in the early stages you can leave, if you find it too ridiculous, but most fans never do. Says Reed, “Anything having to do with [“Doctor Who”], I want.”

Of course, being a dedicated fan has consequences. Naomi Kemp, who adores the band La Dispute, laments, “Imagine how cool I would be if I wouldn’t have found this! I was actually popular before I got into good music!” Austin, an enthusiastic cosplayer, wonders if she’s gone too far “every time I burn myself with my hot glue gun.” And as a result of the avid costume-making and convention-attending, she adds,“I have no more money.”

Getting attached to any kind of entertainment, fictional or otherwise, can be risky business. Like an art student in a museum full of Picassos and Van Goghs, every inch seems like the best thing you’ve ever seen. And then, as your stay in fandom becomes more permanent, you start seeing the parts of the museum that are in disrepair, where your least favorite painting hangs, or where artists you dislike hurt your eyes with their “vision.”

Often art museums have hecklers: you know that one guy who’s there because his friend dragged him, the guy who spends his entire visit saying, “I don’t get it.”

Those are the bad fans.

Those are the fans who think their opinion is the only opinion. And it’s fine to have an opinion – believe me, I have lots – but some fans act like their opinion is law and any other is sacrilegious to the fandom. “The worst kind of fans shame other fans,” says Alannah Danielle, a dedicated Trekkie (a fan of “Star Trek”). “Like, the people who think people aren’t fans because they haven’t seen everything.”

“And then there are the fake fans,” says Bekah Olt, a passionate follower of the band Fall Out Boy. “The ones who think they know everything and try to prove it, but they can’t.”

Everyone I interviewed agreed that the source of certain fans’ extremism isn’t the material; it’s the person. Not every superfan antagonizes others. It’s a slippery slope, both recognizing a toxic fan and becoming a toxic fan. By the time you know for sure, it’s too late.

Luckily, the good fans largely outweigh the bad ones. For every overzealous, defensive fan, there’s someone who’s open to sharing opinions and hearing new perspectives. That’s the core of any fandom: for the fans to experience the same thing and talk about it exhaustively. Fandoms are not a new phenomenon: in 1893, when Sherlock Holmes, the greatest literary detective of all time, was killed off in “The Final Problem,” 20,000 fans canceled their subscription to the magazine that published the short story, and were rumored to wear black armbands to mourn their hero.

People aren’t only fans of television, movies, and books. Fanatics can be history buffs, novice chefs, flamingo statue collectors, or Jelly Belly experts. They can obsess over music, the weather, makeup, cleansing, and cats – sometimes all at once. When people follow their passion, something extraordinary happens; it might make them feel possessive, but it can also let them discuss and share, and sometimes it can even teach them how to really live.

My passion for “Doctor Who” began in 2011, and since then this bizarre world has immersed me to the point of enslavement (to its marketing department, at least). But my interest in the show refueled my interest in television as a whole, and now I’ve set myself on the not-so-fast track to becoming a television producer. Under the right circumstances, the right people can find themselves when they discover something incredible, even if it’s a TV show about time travel, humanity, and an adorkable space alien.

“Doctor Who” led me to my future. What about you?