For several months, a group of teenage girls in upstate New York have been suffering from a mysterious ailment: twitching and verbal tics similar to those seen in Tourette’s patients, but with no apparent organic cause.

Environmental tests have turned up nothing, according to the Le Roy Central School District. A physician with the New York State Department of Health told NBC News that infectious and communicable diseases have also been ruled out.

The diagnosis: “mass psychogenic illness,” or MPI, according to neurologist Laszlo Mechtler, vice president of Dent Neurologic Institute, which has treated 11 of the girls. In MPI, physical symptoms that are perfectly real but that have psychological roots rather than some underlying organic cause appear in a group of people, often spreading from one to the next.

When stress or anxiety manifest as physical symptoms — including twitching, paralysis or other loss of function — in an individual, it’s called a conversion disorder, Mechtler tells the Wall Street Journal's Health Blog.

But when it happens to many people at one time, it crosses into the realm of MPI, he says. Usually that happens when there’s a report or fear of a chemical exposure, toxin or virus — and suddenly people at a school or factory are coughing, sneezing, reporting dizziness or fainting.

The symptoms are believed to arise from emotional reactions to underlying stresses. “These girls are suffering from disabling symptoms,” says Mechtler. “But the issue at hand is that they aren’t caused by a tumor, stroke, infection, drug” or other organic or environmental factor.

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