Nov. 24, 2010 -- Lip-syncing isn't always a faux pas.
At least that's the conclusion 15-year-old Chicago native Keenan Cahill has come to after becoming an Internet sensation through his YouTube channel that features videos of the teen mouthing the words to the top hits.
"I started three years ago, when I got my Mac with a webcam in it," Cahill, who is a sophomore in high school, told ABCNews.com. "I did it to put myself out there and to see what would happen and where it would go."
And where it has gone is viral. Cahill's YouTube channel boasts nearly 200,000 subscribers and has been viewed tens of millions of times. The teen has even appeared on E!'s program "Chelsea Lately" and his taping of 50 Cent's new song "Down on Me" featured a cameo of none other than the famed rapper himself.
"It was a total surprise," said Cahill of 50 Cent's drop-in. "Now this is what I'm known for."
Cahill is also know for a rare genetic disorder that affects the way he looks.
He credits his rise to stardom to pop singer Katy Perry, whose song "Teenage Dream" he featured on his site. Few people had seen the video before Perry herself tweeted, "I heart you" to the teen's Twitter handle in September.
"The numbers started boosting up and up and up," Cahill said.
The video has been viewed more than 18 million times since its Aug. 28 posting. His most recent recording, a lip-sync performance of Will Smith's daughter Willow's hit "Whip My Hair," was viewed more than 200,000 times in one day.
But Cahill has garnered attention not only for his eccentric performances but also because of his unique appearance.
Cahill has a rare genetic disorder known as Maroteaux Lamy Syndrome, which in some cases, including this teen's, results in individuals developing a dwarf-like appearance.
Teen Sensation Keenan Cahill Battles Maroteaux Lamy Syndrome
According to The National Organization for Rare Disorders, Maroteaux Lamy, or MPS Type VI as it's sometimes called, affects one in every 25,000 newborns. The metabolic disorder generally results in growth retardation around age two or three and is progressive, leaving those who suffer from it with "coarsening facial featuries" and "joint stiffness."
Other known complications include strained breathing, blindness, heart abnormalities and a shortened lifespan.
Cahill said he's happy to "let everyone know what disease" he has, and that he knows people are often curious about the way he looks.
"I think I have brought awareness to [the disease]," Cahill said. "A lot of people don't know about it."
Cahill, who said he hopes to become an actor, said he doesn't let his medical setbacks get him down. He's had nine surgeries in his short lifetime, including brain stem surgery a couple of months ago.
"Yeah, that was pretty big," the teen said of the procedure. "But I've had a lot done to me -- hip surgery, a lot of lower surgeries, my hips and my knees.
"I don't talk much about my future," he said. "I just will wait and see what happens."
Every Thursday, Cahill undergoes enzyme infusion therapy, the only FDA-approved treatment of the disorder. A drug called Naglzyme is delivered to Cahill intravenously, and has been shown to improve patients' endurance and generally improve their quality of life.
At an age when many teens tend to feel self-conscious about their appearance, Cahill says he doesn't mind that his YouTube channel makes him an easy target for criticism.
"I've seen all the hurtful comments and you just have to learn to not let anyone get you down about the disease," said Cahill. "I just really don't care what they say."
Keenan Cahill Tells Other Kids to Ignore Their Critics
Barbara Wedehase, the executive director of the National MPS Society, which is the umbrella organization for all mucopolysaccharidose disorders, including Maroteaux Lamy Syndrome, said that other people suffering from rare diseases might be inspired by Cahill's outgoing personality.
"He's 15 and that's an age where people are really scrutinizing their body and looking at other people very closely," Wedehase said. "And to have someone else out there advocating for them just means so much."
The power of the Internet for sufferers of rare disorders should also not be overlooked, she said.
"It's not unusual that a child with a rare disease such as this may be the only child in their community with this disease," Wedehase said. "But to be able to look at Keenan ... he's a terrific role model and can help others with their self-esteem."
But when it comes to inspiring others, Cahill is modest when asked what he'd like to tell other kids facing similar challenges.
"I'm not sure," said the teen. "I think I'd just say to them, don't care what other people think and remember that you're your own person."
"People don't have the right to say, 'hey, don't do this, you can't do this,'" he said. "No matter what people have, they can do anything."