ELMHURST, Ill. (AP) — It started innocently enough, with a boy hamming it up in front of a computer webcam.

He was doing what teens often do when they stand in front of mirrors: Lip-syncing to a popular song, acting goofy, being himself, as if the world wasn’t watching.

Except, in this case, the world was watching when Keenan Cahill — then an impish 13-year-old with a knack for oddball humor — started posting videos of himself on YouTube.

First, there were hundreds of hits, then thousands, then millions. And an Internet sensation was born.

“Keenan, what have you done?” his mom asked when a late-night talk show called and wanted to air one of his lip-syncing videos. She wasn’t sure whether to be amused or horrified.

What if people made fun of her son, who has a rare genetic disease that has stunted his growth? What if he’d opened the door to something too big to handle?

For Keenan, though, this was the adventure he’d been waiting for. This was freedom for a young man whose life had, so far, mainly consisted of spending time at hospitals, when he wasn’t going to school or hanging out in his room.

For him, a vacation was going to Minnesota to a children’s hospital, where he’s had several surgeries on his legs and hips.

Now he gets to travel to places like the Bahamas and France, Los Angeles and Las Vegas. Fans crowd around him as if he were a rock star, their cell phones extended to capture photos and video.

Perhaps most surprising are the celebrities and their representatives who contact him regularly. They want a piece of him, to be seen with him, shoot a video with him, to drink whatever potion there is that equates to online clicks, a new measurement of star power.

And it IS a power, at least right now.

“I finally got somewhere, out of all the people,” says Keenan, who’s now 16. “I used to pray to God. It’s like a switch-back now. I say ‘Thank you’ instead of, ‘Can you please get me there?’”

Whether this newfound fame can last, in an age when online buzz can skyrocket in a matter of hours and fall just as quickly, remains to be seen.

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When Keenan was born in 1995, there was little to indicate that anything was wrong. His parents had noticed that his kneecaps were large, but didn’t think much about it. Then one day, at 6 months of age, his mom remembers Keenan’s face looking puffy, as he sat in his high chair for a feeding.

“Honey, are you OK?” she said aloud.

By age 1, Keenan had fallen off the growth charts. He didn’t walk until he was 18 months old and had unusual fits of vomiting that worried his parents.

They took him to the Mayo Clinic in Minnesota where he underwent several tests. Doctors also sent a skin graft to a lab in Australia for testing. That led them to determine that Keenan had a severe form of an extremely rare disorder called Maroteaux-Lamy Syndrome, also known as MPS, type 6.

People who have the disease are unable to break down complex sugars called mucopolysaccharides, which accumulate in connective tissue and organs throughout the body. Fewer than 1,100 of people worldwide have the disease, which leads to severe disability and a shortened life span in most cases.

Doctors told Keenan’s parents that their son would likely be in a wheelchair by the end of his teenage years if he was not treated. His growth would be extremely hampered, though with this particular type of MPS, there would be no cognitive impairment.

And if he didn’t have a bone marrow transplant early on, he could have died sooner. So in 1997, at age 2, Keenan had that transplant — the first of several procedures and surgeries he has undergone in his young life.

“It was the worst time in my life,” his mom, Erin O’Brien-Cahill, recalls.

After the transplant, Keenan stayed at the hospital for four months and, for a year, couldn’t play with other children or be around other people much to avoid exposing him to illness. His mom had to quit her job as a mortgage underwriter to stay in Minnesota with him. His dad, an electrician, got a job at the University of Minnesota so that he could be with them, too.

Family photos of Keenan show a little boy whose shocks of red hair fell out and eventually came back dark brown as a result of treatments related to the transplant. Over the years, he would grow to his current height of 4-foot-1.

But although he looked different from his peers, he always felt like a regular kid and thrived on making people laugh.

Some have compared Keenan to Gary Coleman, the late actor whose growth was stunted by a kidney disease. But when he’s out in public, people most often stop to tell him that he looks like the young spectacle-wearing actor from the movie “Jerry Maguire.”

Keenan also wears thick glasses that often drop down to the end of his nose. But it’s his smile, sometimes cheesy, sometimes sincere, and his expressive face that draw people in.

The attention made Keenan think about an acting career for himself, gave him something to shoot for.

“I always wanted to be on camera,” he says.

His mom, wanting to give her son something positive to focus on, took him to talent agencies in the Chicago area. He auditioned a few times, but nothing ever came of it.

“Getting noticed is one in a million,” his mom says. “We thought the chances of him achieving his dream were gone.”

Then he got a desktop computer with a webcam for his 13th birthday, and a window opened.

16 Year Old Boy With Rare Disease Becomes Internet Lip Syncing Sensation

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Some might snicker at the notion that Keenan is a star. They might not believe this can really happen to some kid who shoots low-budget videos in his bedroom.

But one only need observe the fervor with which people try to contact him to understand that this phenomenon is, indeed, real.

“It’s completely redefined the definition of celebrity,” says Victor Mehren, a senior marketing director at the Wrigley Co., which hired Cahill to appear in an online advertisement for Juicy Fruit gum in which he stars with a cartoonish singing unicorn.

Keenan hesitates to call himself a celebrity, partly because his manager has advised him not to focus on fame. He’s not even supposed to say the words “celebrity” or “fame.”

“I need to stay grounded,” he says. He talks about backup plans and college. He shrugs nonchalantly when people ask if students at school give him extra attention. “Not really,” he says. “Kids know about it, but they don’t make a big deal.”

Truth be told, though, Keenan is extremely enamored with his new life. He checks his YouTube and Twitter stats. He monitors his cell phone constantly for news about his next public appearances, which he makes on weekends and vacations so he doesn’t miss school. That’s his mom’s rule.

He’s helped open a teen club in the Bahamas. He’s shot a commercial with actress Jennifer Aniston, which has gotten more than 8 million hits. He’s done videos with rapper 50 Cent and “Jersey Shore” star DJ Pauly D, among others.

He’s also joked comfortably with comedian Chelsea Handler on her late-night talk show, giving her his phone number and calling her a “cougar,” a teasing reference to women who go after younger men.

And pop star Katy Perry sent him a video greeting with other celebrities included when Keenan recently turned 16.

Perry, whom Keenan has never met, is the one he calls his “mentor,” the one who sent his life into an upward spiral last fall with a simple posting on Twitter, after she saw a video of him lip-syncing to her song “Teenage Dream.”

“I heart you (at)KeenanCahill,” the tweet read.

Keenan had already been getting attention, but this took it to a new level. The e-mails and phone calls poured in.

It was too much to handle. So when David Graham and Mark Long, who got their start in reality TV, approached Keenan and his mom about being on one of their shows, his mom asked if they would be his managers.

“When Mark and I got Keenan, we said, ‘OK, we’re going to set this kid up right from day one,” says Graham, who’s based in Las Vegas.

No drinking and no partying, they said, setting rules that were no problem for a kid who seems to have no interest in that, anyway.

And no appearances on B-level talk shows. “He doesn’t need that, doesn’t need to be overexposed,” says Graham, who’s focused more on videos with celebrities, ad deals and club appearances. Several of the artists Keenan has done videos with work with EMI Records, which is Perry’s label. Keenan also has recorded his own song — one in which he actually sings — set for release this spring or in early summer.

It sounds like it might be lucrative, but the financial rewards are not that great, Graham says.

“He’s not going to be able to retire off this. This is more or less a cool after-school job,” he says. “For him to really cash in, he’d have to land a TV show or a movie.”

That’s Keenan’s big hope, that this attention will lead to a sitcom. But Graham isn’t sure that will happen.

“I don’t want Keenan to go to LA and experience failure. I want him to come there, shoot something and go home,” Graham says. “I don’t want him to have a TV show that fails and have him go back to school and everyone makes fun of him.”

During the telephone interview, Graham pauses a few times to answer messages from Keenan, who is texting on his lunch hour at school.

“It’s like having a child,” Graham says. “He’s constantly asking questions.”

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Keenan’s mom, who’s back working in the mortgage insurance industry, continues to shake her head at all this, though usually with a smile on her face. Her boy is happy, and that makes her happy.

“It’s insanity,” she says, “but a good insanity.”

Even she can’t quite wrap her head around the mania surrounding her son when he lip-syncs at live performances.

“You’d think Barbra Streisand was singing. It’s like, really? For real?” she says as she sits in the suburban Chicago town house she and Keenan share with his younger sister. Keenan’s parents divorced five years ago, though his dad still accompanies him to some of his engagements.

It is, indeed, quite a heady and sometimes bizarre scene for a 16-year-old. Keenan is whisked in and out of nightclubs, where he usually can’t stay too long because he’s underage. He signs autographs. Poses for photos with Playboy bunnies in Las Vegas, baseball players at spring training in Arizona, people on the street who stop him.

One club in Israel hosted a “Keenan Cahill Worshipping Party” late last year. Keenan wasn’t able to attend, so partygoers danced with posters of him, instead.

“He’s amazing. He’s so sexy. I love him!” one of them said in a video recorded at the event.

“I think he’s one of the most important artists of our time,” said another.

The party’s tongue-in-cheek tone hinted that Keenan was more a curiosity, almost a pop culture caricature of himself.

Some would call it playful fun.

But it’s gone well beyond that on his YouTube page, where the comments are sometimes brazen, and harsh. People have called him everything from a “rent-a-dwarf” to a “freak,” and worse, though those comments often send his fans rushing in to defend him.

Others question whether the celebrities who do videos with Keenan are really just using a naive teenager, one who has a disability no less.

One could argue, though, that it was Keenan who originally used a lot of artists’ songs, without permission, to get famous. So it begs the question: Who’s using whom?

“We think that about child stars: ‘Oh, he’s being used. He’s being abused. People are taking advantage of him.’ Why that hasn’t been a problem so far (with Keenan) is that he seems to be enjoying himself,” says Kelly O’Keefe, a professor of brand management at Virginia Commonwealth University.

“He put himself out there. Now he’s living a dream and getting to meet celebrities.”

Keenan would agree. He sees some of the mean comments online, but just shrugs.

“I think it comes from my mom always telling me, ‘You’re normal,’” Keenan says. It doesn’t really bother him.

The fun he’s having also has made dealing with the realities of his medical condition easier.

This summer, he’ll have yet another surgery to reconstruct a hip and straighten a leg. It’ll mean that he can’t make appearances for several weeks while he recovers. Other procedures he’s undergone include surgeries to staple the growth plate in his knees to straighten them and another to decompress his brain stem.

Each Thursday, he also gets a four-hour infusion, called enzyme replacement therapy, at home or at Children’s Memorial Hospital in Chicago.

Keenan is among the first wave of patients with MPS 6 to receive these infusions, which seem to slow the progression of the disease.

“We can’t reverse necessarily all of the damage to various organs,” says Dr. Barbara K. Burton, a clinical geneticist who heads the MPS treatment program at Children’s Memorial. “But the hope is that we stabilize the disorder and increase life expectancy.”

Without treatment, she says, life expectancy for patients with MPS 6 is less than 20 years.

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So there are many questions about what Keenan’s future holds. How long will he live? Will he be able to stay out of a wheelchair? How long will this YouTube ride last?

Few hold out hope that the latter will last too long.

“These phenomena don’t have forever staying power. At some point, his 15 minutes of fame may well be over, and that will feel like a letdown,” says O’Keefe at VCU.

Graham, Keenan’s manager, says he’d be happy if it lasted through high school and long enough for Keenan to earn money for a special car that allows people who aren’t very tall to drive.

“Keenan’s one of the pioneers,” Graham says of YouTube fame. “But viral is a notch below reality television, as far as how society views it.”

His mom says, “Whatever it is will be OK,” though she knows Keenan clearly still has his hopes up.

“I don’t like talking about the end,” he admits.

With that, he asks if he can be excused from an interview. “Are we done?”

His mom tells him to stay put, but he stands up, flashes that cheesy smile and slowly sidesteps away, so he can go back upstairs to his bedroom.

And his computer.

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Online:

Keenan’s website: http://www.keenansroom.com

Birthday video: http://bit.ly/gnrQbU

Aniston commercial: http://bit.ly/hiZO6w

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Martha Irvine is an AP national writer. She can be reached at mirvine(at)ap.org