A Channel 4 documentary has examined how a small town in the US appeared to "catch" Tourette's syndrome.
The film, The Town that Caught Tourette's, focuses on the community of Le Roy, New York, where more than a dozen teenage girls developed Tourette's-like symptoms, such as verbal outbursts, twitching and limb gestures.
Doctors who saw the girls believed they were victims of conversion disorder, where real symptoms are triggered by a psychological trauma.
Panicked parents said their daughters were not suffering from any mental problems, as they were high achievers from stable backgrounds.
However, in the months that followed, 18 students became ill and doctors said it was a case of "mass hysteria", where symptoms spread among vulnerable people who are in close proximity.
The girls went on national TV to tell their story. Doctors claimed their symptoms had been magnified because they had seen other Tourette's sufferers on TV and on social networking sites.
In an interview with the Sun, several of the girls spoke about their experiences. Katie Krautwurst, 16, said she started twitching while she was at her boyfriend's house. "Some days I just started twitching and it wouldn't stop for 20 minutes," she said.
Thera Sanchez, 17, added: "I used to cheer every day. But it got so bad I had to stop going to school."
After locals wrote hundreds of letters, famous environmental activist Erin Brockovich investigated and found there was a train crash in 1971, which had spilled gallons of industrial solvent into the town's water supply.
At its peak, even more people came forward saying they had symptoms of the disease, including a 36-year-old woman who lived 300 miles away.
However, as soon as the media arrived, the symptoms eased and the girls started getting better. It also emerged that many of the teenagers had experienced horrific trauma, abuse and family misfortunes. It is thought these problems may have caused the Tourette's all along, though not everyone agrees.
Doctors said the problem may have been caused by an infection called Paediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcus. After being using antibiotics, the symptoms began to disappear.
Krautwurst is quoted as saying: "I still can't explain how we all got the same problems. The doctors said it was traumatic, but I really don't think any of us had that traumatic a life before it happened."