WASHINGTON – Leah Packer was a medical mystery.

For several months, her parents ferried their 13-year-old daughter from one emergency room to another, looking on with fear as she doubled over in stomach pain.

Last summer with no warning, the middle school student from Olney, Md., started having spells: she would vomit just about everything she ate and drank.

By September, Leah, a cross-country runner and soccer player, found she could no longer keep up with the other kids her age.

“I started feeling very tired, had achy joints