ABU DHABI // It started late last year among a group of teenagers at a high school in upstate New York.
Without warning, each began suffering from involuntary tics, stammering and verbal outbursts similar to those brought on by Tourette's syndrome.
Then others emerged with the same symptoms, including a woman unconnected with the school.
Rumours soon started to circulate, linking the outbreak to causes including drug use and medical vaccination.
And now Erin Brockovich - the environmentalist whose story was turned into the eponymous, Oscar-winning movie with Julia Roberts - is on the case, claiming an unlikely link with a toxic chemical released in a train accident more than 40 years ago.
But doctors and some of the victims already think they have identified the true culprit. Some call it "conversion disorder", while others are using the term "MPI": mass psychogenic illness. Yet it is known best by its most provocative name - mass hysteria.
Derived from the Greek for uterus, the term is a throwback to the Dark Ages when doctors believed the condition had its origins in the womb.
Even the more neutral term "conversion disorder" harks back to largely discredited theories developed by Sigmund Freud.
What is beyond dispute is that medical science is still struggling to explain a condition now known to affect people of either sex and any age.
One of its most perplexing features is its apparent infectiousness. During the 18th century, doctors in England were stunned by an outbreak of mass hysteria at a cotton mill in Lancashire, which began when a girl dropped a mouse on a nervous colleague.
The victim instantly went into convulsions - to be followed a day later by many of her colleagues, who developed the same symptoms. Within 48 hours so many had been "infected" that the whole factory was brought to a standstill.
Only after a doctor had taken the drastic step of applying electric shocks to the victims did the symptoms disappear.
The sheer range of manifestations is no less astonishing. In 1963, more than 1,000 Kenyan villagers around Lake Victoria were gripped by laughing fits for up to a fortnight or more.
A few years later, Singapore was hit by an epidemic of men convinced their genitalia were shrinking.
Understanding the real causes of such outbreaks would do more than just tidy up a medical mystery. Even today, many of those afflicted resent the insistence of doctors that the cause is "all in the mind".
Such resentment is understandable, given that the symptoms are certainly real enough. But new research makes such a dismissive attitude even harder to sustain, by showing that those suffering from the condition may have brains wired up slightly differently to the rest of us.
At the United States National Institutes of Health in Bethesda, a team led by Dr Valerie Voon has been using brain-scanning technology to investigate one of the most striking features of the condition experienced by the New York teenagers: the way in which a mental state turns into involuntary movements in what specialists call motor-conversion disorder.
In effect, the team is exploring the key issue behind the ancient belief in "dualism": how the supposed schism between the immaterial mind and the physical body is bridged.
To find out, Dr Voon and her colleagues recruited 16 male and female patients with the disorder, along with a control group of 16 matched but unaffected people.
Previous research suggested that regions of the brain known as the amygdalae might be involved in converting emotional states into physical responses.
These small collections of nerve cells deep in the brain are known to play a crucial role in judging incoming sensory signals and deciding how to respond to them.
In normal people, the amygdalae quickly become inured to the same sensory input, allowing the brain to stay alert to fresh signals. But brain scanning of the patients with motor-conversion disorder revealed that their amygdalae were slower to lose interest in old stimuli - leaving them vulnerable to overreaction.
These same patients also had unusually strong connections between their amygdalae and the parts of the brain responsible for movement.
Taken together, the findings suggest very subtle flaws in the structure of the brain can lead to over-response to stimuli, which manifests itself as inappropriate physical movements.
Such research may prove useful to the doctors trying to help the teenagers in New York. Perhaps if they are exposed to some other stimulus, it may help their amygdalae snap out of their fixated state and resume normal activity. Perhaps that's what the electric shock therapy did to those suffering from the condition in that 18th-century Lancashire mill.
But this new research may also cast light on perhaps the most bizarre manifestation of motor conversion disorder: dancing mania.
In the Middle Ages, European cities experienced outbreaks of uncontrolled dancing by crowds of people.
In July 1518, the French town of Strasbourg was struck by a "dance epidemic", with hundreds of people gyrating so wildly that some died of exhaustion.
A clue as to what might have triggered such mass outbreaks of motor-conversion disorder may come from recent research into the human response to music. We all know the feeling of listening to rhythmic music and finding ourselves tapping along to it. Scientists at the International Laboratory for Brain, Music and Sound Research in Montreal have found that music can cause this response by stimulating certain parts of the brain, including the amygdalae, the regions implicated in conversion disorder.
This raises an intriguing possibility for curing those teenagers in New York. Perhaps their amygdalae could be reset by exposing them to some good, old-fashioned pop music.
It may just work - and it will certainly be more pleasant than electric-shock therapy.
Robert Matthews is a visiting reader in science at Aston University, Birmingham, England