BOSTON -- Most of us get hungry. We eat. We feel full. Now, imagine always being hungry, and no matter how much food you ingest, you never feel satisfied.

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The feeling can be so strong that food must literally be locked away.

NewsCenter 5's Medical Editor Dr. Timothy Johnson said that Max Rivera came into the world with weak muscles and little strength. He also had trouble eating. But as he grew older, he gradually developed the opposite problem, an insatiable appetite.

"When he was born he was very floppy, just like Jell-O," Max's mother, Charlotte Herrmann, said.

Today at age 7, Max could easily eat all day and all night, without ever feeling full.

"Weight gain is the big issue," Herrmann said. "He is hungry all the time and he has a low-calorie diet and we have to be very careful with his food."

Like one in 12,000 children, Max has Prader-Willi Syndrome.

But unlike most who overeat, Max's urge is strictly genetic -- a defect in one of his chromosomes has programmed him to eat all the time. If the diet goes uncontrolled, children with this syndrome can die at a young age -- in their teens or 20s.

"The children when they grow up can have all the problems that people with a higher weight have. They have a tendency to diabetes. The weight itself and the muscle weakness can be totally incapacitating," Massachusetts General Hospital Dr. Lynne Levitsky said.

Controlling the diet can, in severe cases, mean physically locking up food -- cabinets and refrigerators. Max's mom keeps food hidden, or out of reach, but still, he manages to find it.

"He will often get up in the night and have a snack because he's hungry," Herrmann said. "He will take food and hide it under his pillow."

Making the condition even harder to manage -- children with this syndrome also tend to have behavioral problems. Max also has a learning disability.

Max's lunch is carefully packed each day -- restricting him to 1,000 calories. Max also takes daily shots of growth hormone, which can increase muscle tone and decrease body fat.

Max's mom hopes that a new public service campaign will help people understand Prader-Willi syndrom and not judge children by how much they eat or they weigh, but by who they are inside.

"He's a very, very bright child. He's very verbal, talkative and very funny," Herrmann said. "He's a real character. He's the joy of my life."