Before the media vans took over Main Street, before the environmental testers came to dig at the soil, before the doctor came to take blood, before strangers started knocking on doors and asking question after question, Katie Krautwurst, a high-school cheerleader from Le Roy, N.Y., woke up from a nap. Instantly, she knew something was wrong. Her chin was jutting forward uncontrollably and her face was contracting into spasms. She was still twitching a few weeks later when her best friend, Thera Sanchez, captain of one of the school’s cheerleading squads, awoke from a nap stuttering and then later started twitching, her arms flailing and head jerking. Two weeks after that, Lydia Parker, also a senior, erupted in tics and arm swings and hums. Then word got around that Chelsey Dumars, another cheerleader, who recently moved to town, was making the same strange noises, the same strange movements, leaving school early on the days she could make it to class at all. The numbers grew — 12, then 16, then 18, in a school of 600 — and as they swelled, the ranks of the sufferers came to include a wider swath of the Le Roy high-school hierarchy: girls who weren’t cheerleaders, girls who kept to themselves and had studs in their lips. There was even one boy and an older woman, age 36. Parents wept as their daughters stuttered at the dinner table. Teachers shut their classroom doors when they heard a din of outbursts, one cry triggering another, sending the increasingly familiar sounds ricocheting through the halls. Within a few months, as the camera crews continued to descend, the community barely seemed to recognize itself. One expert after another arrived to pontificate about what was wrong in Le Roy, a town of 7,500 in Western New York that had long prided itself on the things it got right. The kids here were wholesome and happy, their parents insisted — “cheerleaders and honor students,” as one father said — products of a place that, while not perfect, was made up more of what was good about small-town America than what was bad. Now, though, the girls’ writhing and stuttering suggested something troubling, either arising from within the community or being perpetrated on it, a mystery that proved irresistible for onlookers, whose attention would soon become part of the story itself.

Le Roy’s East Main Street displays an impressive row of grand Victorians and Federalist-style homes built in the 19th century, testament to the flour mills and salt mines that made the town a comfortable place to live. After that came the Jell-O years, when that company and several others employed thousands of people in the area. But Jell-O and most of the rest of the factories took their work elsewhere by the 1960s, and now a good number of those historic homes have been divided into two- or three-family rentals, with peeling paint and rows of crooked mailboxes inside the foyer. Some houses look so beaten down by weather and disrepair that it comes as a surprise to see a light on inside. Le Roy is a working-class community with good schools that attract people who work in nearby Rochester. But it is also a manufacturing town whose prosperous days are behind it — the kind of place where local politicians are always talking about how to bring back the good old days.

Katie Krautwurst and her family live in one of the stately, well-preserved houses in town, a home her mother’s ancestors built, its porch now decorated with semicircular American flag banners and a child’s antique sled. At the top of a winding staircase is Katie’s room, a pink-and-yellow perch where she and Thera sat talking one late February afternoon. The girls grew close a few years ago, when they met through cheerleading and realized they both had a crush on the same guy. “How weird was that?” Thera asked, her voice going loud and her eyes going wide. Thera speaks in italics and underlines; Katie, by contrast, is so reserved she could be mistaken for nonchalant. As they talked, Thera was idly going through Katie’s walk-in closet to see what was new. Katie’s face showed a quick spasm, a twitch, every few minutes, subtle enough that you might miss it if you weren’t looking. Thera had a bruise on her left leg from where she had fainted the evening before and landed on her bedside table.

On the afternoon when Katie first started twitching, she was at her boyfriend’s house. When the symptoms worsened, his mother called her mother, who told them to call an ambulance and meet her at the emergency room in Rochester. Paramedics strapped Katie onto the stretcher. “Then I couldn’t twitch, so it made it even worse, and I was freaking out even more,” she said. Doctors at the hospital told Katie and her mother that she was having an anxiety attack. Katie was a straight-A student who admits she can be anxious at times. But her symptoms persisted, so she and her mother went back to the same emergency room a few days later. This time, Katie’s mother, Beth Miller, a nurse, insisted they conduct more elaborate tests. After seven hours of testing that included an M.R.I. and a blood panel, the doctors told Miller what she already knew: her daughter had tics.