How did this outbreak start?
In October, several cheerleaders at a high school in Le Roy, a town of 7,600 in western New York, began exhibiting unexplained symptoms similar to those of Tourette's syndrome — facial spasms, involuntary arm swings, stuttering, and sudden verbal outbursts. The ranks of sufferers soon swelled to at least 16 girls, along with a teenage boy and a 36-year-old woman. At first, the cases baffled doctors and caused frightened speculation among parents, who blamed rare strep infections, side effects from a vaccine against the human papilloma­virus, and toxins left by a chemical spill near the school four decades ago. Environmental activist Erin Brockovich was even called in to investigate. But tests have so far ruled out all environmental and infectious causes. The leading theory now is that the girls suffer from what is known as "conversion disorder."
What is conversion disorder?
It's a psychological phenomenon in which people convert stress or emotional trauma into physical symptoms, sometimes in response to witnessing those symptoms in others. The disorder remains poorly understood, but the symptoms are very real and can appear suddenly in otherwise healthy people. Experts say it is a matter of the brain making the body sick, as it does when stage fright causes nausea or breathing problems. When multiple people are affected, experts refer to it as mass psychogenic illness, or what used to be known as mass hysteria, a term that has fallen out of favor because of its sexist connotations. These outbreaks, which typically affect girls more than boys, are believed to "spread" from person to person via unconscious social mimicry of other people, just as watching someone yawn can cause others to do so involuntarily. The girls in Le Roy "come from a small community," said Dr. Laszlo Mechtler, who is treating some of them. "One may have had a significant symptom, and it was like a wildfire."
Do such outbreaks happen often?
More often than people realize. In 2007, more than a dozen girls in a Florida high school developed unexplained wheezing that lasted weeks; the same year, 600 girls at a Catholic boarding school in Mexico experienced mysterious fevers and buckling knees. In 2010, at least 15 students at two all-girls schools in Brunei began screaming and shaking uncontrollably. Outbreaks have been recorded not just in schools, but also in workplaces over the past few centuries. In 1789, a female worker at an English textile factory put a mouse down the dress of a skittish co-worker, who began to convulse. Workers began to whisper that imported cotton had caused the reaction, and the factory had to be shut down after 24 other workers suffered similar convulsions. More recently, in 1977, dozens of female workers at a West Virginia shoe factory fainted on the job; they said they were overcome with toxic fumes, but federal investigators diagnosed them with "assembly-line hysteria." Similar outbreaks occurred in convents during the Middle Ages.

Why are girls more susceptible?
There are several possible reasons. Girls are socialized to suppress stress, rather than act out aggressively, causing it to manifest in otherwise unexplained physical symptoms. During adolescence, girls also have an acute desire to belong to a group, which can add to their stress and make them especially attentive to social cues. "Teenage girls are very susceptible to drama and each other's moods," says clinical psychologist Nancy Molitor. And women are more likely to seek medical treatment than men, which may skew the numbers involved in mass outbreaks.

Were the girls in Le Roy under stress?
The girls have repeatedly claimed that they were normal, happy teenagers before their symptoms appeared. But Dr. Jennifer McVige, a local pediatric neurologist, says that all 10 of the girls she saw "had something big that happened" in their lives prior to the outbreak, whether it was a divorce, a sick parent, abuse, or other emotionally upsetting event. It has also emerged that three of the girls suffered from tic disorders prior to the outbreak, which doctors say could have unconsciously influenced the other patients; those who came down with symptoms posted videos of themselves twitching on Facebook, which other girls viewed before developing the same symptoms themselves. Nonetheless, several of the girls' parents, and many other people in the Le Roy community, are skeptical about the diagnosis of conversion disorder, and have called for more tests aimed at finding possible environmental and medical factors.
Are they getting better?
Reports indicate that they are. Symp­toms have disappeared altogether for several girls, and others are being treated with a combination of anti-anxiety drugs, cognitive-behavioral therapy, and counseling. Doctors report that their conditions have steadily improved, particularly after videos of the girls twitching were removed from TV stations' websites and social media. "These girls will get better," says Mechtler. "We have to give them time and space."

The susceptibility of cheerleaders
It hasn't escaped researchers' notice that cheerleaders often figure in cases of mass psychogenic outbreaks. Four of the first girls to exhibit symptoms in Le Roy were on the cheerleading squad. In 2002, 10 girls began suffering unexplained seizures at a school in rural North Carolina; half were cheerleaders. And in 1952, scores of members of a Louisiana cheerleading squad mysteriously fainted in quick succession at a football game. Experts theorize that because cheerleaders are considered popular, outbreaks are more likely to begin with them than with classmates of lower status. There's also a theory that cohesive groups, particularly those that wear matching uniforms and excel at synchronized movements, might be more susceptible to mass suggestion. But that's as close as social psychologists have gotten to figuring out why, as social critic Caitlin Flanagan recently wrote, "it is the cheerleaders and not the linebackers who come down with tics and stuttering."