When Mathilda Crisp was about three years-old, she stopped sleeping through the night. But during the day, she would fall asleep without warning — during a swim lesson, in the middle of her cereal bowl at breakfast.

Then other, stranger symptoms started materializing: when she got happy or emotional, she would suddenly collapse. (Her brother and sister started carrying her around the house on a chair so she could keep playing in their games.) She would thrust her tongue around her mouth. She couldn’t seem to walk in a straight line.

At first her doctors were sure she had a brain tumor. But her scans were negative. They tested her for leukemia, Lyme disease. Nothing. But when one doctor finally did diagnose Mathilda, it turned out to be just the beginning of an even bigger mystery: of why this little girl — and a handful of other kids in Northern Europe — had suddenly been struck ill. Trying to solve it has become one doctor’s life’s work.

Also check out:

• Mathilda’s mom has written a book about her experience. You can read an exclusive excerpt from it, or check it out on Kindle.
• Dr. Emmanuel Mignot is a key player in this story. He first got media attention for his colony of narcoleptic dogs. Check them out.
• While researching this story, Mary learned a lot about the flu vaccine. Here are her top five takeaways.