It's a medical mystery that has baffled experts and fascinated just about everyone else for months.

About four months ago, 12 girls at the same high school in New York State all began suffering strange symptoms, similar to Tourette's syndrome. The girls have been enduring uncontrollable body movements, facial tics and verbal outbursts.

The girls' parents wondered whether an infectious agent had caused the "outbreak," while others wondered there was something in the air or the environment of the school.

After a three-month investigation, state health officials ruled out carbon monoxide, mould, illegal drugs and a host of other possible causes.

Now, doctors say they think they know what's causing the symptoms: conversion disorder.

Dr. Jennifer McVige, a pediatric neurologist at the Dent Neurologic Institute, says they made the diagnosis after conducting blood and other lab tests, brain imaging tests, and reviewing the environmental study reports.

"This is a diagnosis of exclusion. So, when everything else is ruled out, this is what we decide upon," she explained to CTV's Canada AM from Buffalo, N.Y. Thursday.

Dr. McVige is treating 11 of the 12 patients in this case, but notes she has only been given permission to talk about four of them.

"So by no means am I saying that this is everybody's diagnosis," she said.

Conversion disorder is a relatively rare condition that, in another time, might have been called an epidemic of "hysteria." It's a stress-induced psychological condition that triggers a physical reaction within the body.

"So if you have stress inside of you, it wells up inside of you and comes out in some physical manner, be it a tic, or seizure, or fainting spells, or blindness," McVige said.

While some have accused the girls of faking their symptoms in a bid for attention, McVige insists that's not possible.

"Oh no, I have been a very busy woman and this is not a hoax. I wish for the sake of the girls that I could say that it wasn't real. It's been very difficult for everyone involved," she said.

"…They are not faking the symptoms. This is not something they are doing purposefully. They didn't even know each other prior to this, besides just being in the same school."

McVige says what sets off the condition is a bit of a mystery.

"It's difficult to say what causes it. Oftentimes there's a certain stressor that sets off the symptoms. So part of the treatment is to figure out what the stressor is, acknowledge it, talk to the person about it," she said.

The treatment includes behaviour modification therapies to manage the actual tics, psychotherapy to help with the root cause, and sometimes medication and muscle relaxants to alleviate spasm.

All the girls are being treated and are said to be improving.