The last time Shawn Cooper was craving sugar she ate an entire jar of jelly. Then she had some apple butter, and maple syrup too. Shawn isn't a toddler with an overactive sweet tooth; she is a woman living with Prader -Willi syndrome (PWS). Prader-Willi syndrome causes a constant sense of hunger coupled with a drastically slower metabolism. It is the leading genetic cause of obesity in children today, according to PWS Association.

Shawn, 35, is literally always hungry. Her condition requires around-the-clock vigilance and years of self-discipline to keep obesity at bay. She was born with a deletion in one part of chromosome 15 that came from her father. The missing or turned off gene that cause PWS is fairly rare, with about 30,000 cases in the U.S. For every 10 babies born with Down syndrome, one child will be born with PWS.

Dottie Cooper, Shawn's mother, was in the dark for nearly a decade about what was wrong with her daughter. Dottie remembers the baby being almost immobile in the womb, barely kicking at all. She was only three weeks premature in November 1973, but at barely five pounds. The doctors asked if Dottie was sure the baby wasn't conceived earlier. Shawn's muscle tone was so weak that her shoulders drooped back over her mother's arm.

At first, it was her lack of appetite that was the problem. The newborn barely ate because she wasn't strong enough to suck milk. Today, many PWS babies are put on feeding tubes. Shawn ate from an eyedropper until she could take a preemie nipple. "We learned every small step for [a] baby was one giant step for Shawn," says Dottie. "We celebrated the little victories and we've done that all through our lives."

And slowly, victories came. Shawn's low muscle tone and small stature meant that she scooted and rolled on the floor at the time most children learn to walk. Finally, around 3 years old, she got on her feet. Even as the toddler got stronger, Dottie and her husband Dale carted Shawn to endless specialists. No one knew what was wrong with their daughter.

Not only did Shawn start walking, she also started eating. "Once it starts, it becomes very ravenous," says Merlin Butler, MD, PhD, FFACMG, professor of psychiatry, behavioral sciences and pediatrics at Kansas University Medical Center and a leading expert on PWS. "It's like 'oh great, now the baby is eating!' But then it goes from an interest to the extreme." People with PWS never get the full feeling that most of us get after eating. In brain scans Butler says that PWS patients actually get more stimulated when they see food after eating, creating a vicious cycle where food just fuels the desire for more food.

At 4 years old, Shawn was "real chunky." By the time she was school-aged, Dottie had to limit what she ate. The workers in the cafeteria were told not to let her buy anything. She had to sit with the teachers because other kids would slip her food. "It was hard," Shawn admits. "[Mom] would tell me and I'd end up eating the wrong thing at the wrong time. I'd be in tears because I was still hungry and I made a mistake."

At home, she had to be watched all the time. For a while, there was a padlock on the fridge. "I would find a way to get around it," Shawn says. "I'd find a way to get food."

The Coopers stopped just short of locking Shawn in her room at night. She was nearly 100 pounds at age 10, despite being far shorter than other kids. Around that time a friend of Dottie's dropped off an article, "Children With Rare Disorder Literally Eat Themselves to Death."

"It was like describing my daughter," says Dottie. The article was about Marge Wett's daughter, Lisa, who also had PWS. Dottie picked up the phone and called Wett, who was the first executive director of the Prader-Willi Syndrome Association in the U.S. "It was so comforting to talk to [the] one person in the world who knew what I was going through."

Dottie ordered a pack of materials that contained nearly all that was known about the disease at the time. Although the syndrome was first described in the 1950s, the genetics were not unraveled until the 1980s. By then, the age for diagnosis was about six years old. The average life span was the early 20s, Dottie read. "That excitement turned to a feeling of being afraid and very sad," she says.

As Dottie started attending conferences, getting more information and also some peace of mind, Shawn continued to eat. Every day was a struggle. Instead of locking down food, the Coopers began rewarding Shawn for not getting into food. Once a week she was treated with a Diet Coke. She joined Weight Watchers at 10 years old and dropped from 99 down to 88 pounds at 11.

Because people with PWS have low muscle tone, their metabolism is lethargic at best. A patient with PWS might only need 1,000 to 1,2000 a day to maintain their weight. Shawn ate about 900 calories daily throughout her teens. She plateaued at about 130, overweight but not obese for a woman who is 4-foot-10]. However, the slip-ups continued.

Shawn's brother Chad brought home a fruitcake for their mother on Christmas nearly a decade ago. The large cake, soaked in brandy, was so heavy a small slice was all you could eat at once, Dottie remembers. She set it to the side out of Shawn's sight, but not far enough. "I love cake," Shawn admits.

The entire cake, enough to last a week, was gone. People with PWS lack the ability to vomit, and so Dottie wasn't just angry, she was scared. She watched Shawn carefully to make sure that she didn't complain of any stomach problems. Overeating can mean death if the stomach ruptures. Even worse, PWS patients have a high threshold for pain, so a fatal stomach injury may not even elicit a complaint.

There are other problems that many people with PWS have. Shawn recently found out she has an adrenal insufficiency, so her body can't handle stress well. Many patients have sleep apnea, are obsessive compulsive and pick at their skin. Because most patients are overweight or obese, all the problems that come with obesity, from cardiovascular problems to diabetes, are also a problem for those with PWS.

Despite all the other health issues, it is eating that is the hallmark of this condition. "Recently there was a report of [someone eating] sanitizers. There was [someone with] Prader-Willi that unscrewed one of those dispensers because it was vanilla-scented," says Butler. "Dog food, cat food, fish in aquarium, frozen hamburger; it's all game. Anything that resembles food."

Today, Shawn claims she would never eat the entire fruitcake. "Sometimes I will slip, but I try not to," she says. "I do try some exercising to make up for it." Shawn hits the gym at the YMCA five days a week near her home in Atlanta. She works out with a personal trainer twice a week and swims, weight trains, and walks on the treadmill. She eats just three small meals and two snacks a day and weighs in weekly. After a few years of creeping up to almost 190 pounds, she is trying to get back down to 130. So far she's lost 31 pounds.

For children born with PWS today, some have it a little easier. Babies are usually diagnosed at months, not years of age. Children are often put on growth hormones, sometimes as young as 6 months old. Kids with PWS can grow to be nearly as tall as other family members, with improved muscle tone that translates into a higher metabolism.

But ultimately their struggle will be the same as Shawn's and the other 400,000 patients worldwide diagnosed with PWS. She has outbursts less often than she did in her teens, and she's learned to live with the condition as best she can. However a caregiver still comes to the house most days of the week. "Managing someone with Prader-Willi is a full time job," says Dottie. "It doesn't matter how energetic and dedicated you are –- everyone needs a break."

To keep herself in check she says, "I'm full" out loud after every meal to send a message to her brain, since her body does not do it for her. But even with the endless vigilance, she will still eat. "It can happen anytime," she says. "It wil