As The Early Show correspondent Melinda Murphy reports for a two-part series, there has to be a trigger to set the disease into motion. For some, it can be a simple case of a childhood disease such as strep throat.
Just last week, 7-year-old Kennedy Pieken was in the hospital.
"She got the flu over the weekend and ended up losing five pounds and getting very dehydrated. She wouldn't eat or drink anything," explains her mother, Jodi Pieken.
Kennedy had lost five pounds, something she couldn't afford, because this first grader from Lake View, Iowa, has been battling anorexia for three years.
"The way she was going, I was afraid she was gonna die. I mean, her hair was falling out. She looked awful," says Jodi.
Kennedy's parents didn't know what to do when their preschooler suddenly stopped eating.
"I would wake up in the morning hoping she was still gonna be alive and then thinking, 'OK, now I gotta start the day over again. Trying to get her to eat.' It was awful," Jodi explains.
And although Kennedy was only 4, she knew exactly what she was doing.
"I nibbled on it a little bit. and then I pretended to eat it. So then I pretended to swallow it, too," says Kennedy.
She admits she would put the food in her cheeks and then spit it out later.
For five months, Kennedy lived on ice cream, chocolate milk and pudding. Even getting those foods down was a struggle.
"Even when she would eat the pudding it would just be like sticking the spoon into the pudding, getting a dab of it, and then putting it in her mouth that way. You know, it would take her almost two hours to eat a little cup of pudding," Kennedy's father, Roger Pieken, remembers.
"There was one point, Roger was holding her with her mouth open. And I was trying to shove a tiny bite of peach into her mouth. She was screaming. Bawling," says Jodi.
Asked what caused her to stop eating, Kennedy says, "I don't know. Because my brain was telling me not to eat."
Jodi realized this wasn't just a case of picky eating and began to suspect anorexia; she knew the signs, because she has fought the disease herself for years
"This summer, actually, I was in Omaha, at a clinic for three weeks with it. Finally, my husband and a friend of mine said, 'You need help. You have to do something.' Cause I got under 100 (pounds)," explains Jodi.