One snowy afternoon in October 1985, eight children from the tiny farming community of Lyndonville, N.Y., went sledding together. Within a few weeks, they all got sick.

David Bell, the local doctor who treated the children, recalls that their symptoms were similar to the flu: sore throats, fevers, muscle aches and severe fatigue. After three days, they hadn't recovered. Then a week. A month. Ninety days.

Six months after their sledding trip, the children still couldn't go back to the lone school in town. They had trouble getting out of bed. Light gave them a headache. Four of the eight were so sick that they were essentially disabled, Dr. Bell recalls. Tests ruled out mono and other infections. "We had no idea at all what it was," he says.

Over the next two years, the mysterious illness spread throughout this rural village of 862 people halfway between Buffalo and Rochester. It eventually affected 214 people within a 30-mile radius, 46 of them children.

For the next 25 years, Dr. Bell, a Boston University-trained physician, would collaborate on studies, maintain a vast database of patients and write books, searching in vain for the cause of the illness from which some of his patients recovered and many did not. The illness would eventually be identified as chronic fatigue syndrome, but scientists still have not found its cause. "I figured I would never know why the kids got sick," Dr. Bell says.