ELMHURST, Ill. -- 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.

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.