On September 26, a student at William Byrd high school in the Blue Ridge Mountains of Virginia reported suffering twitching, tremors, dizziness and headaches. Days afterwards, another student announced the same symptoms. Later in the month, more students from the school in Roanoke County were seen twitching. "They wave. It's convulsing. They can't stop it," student Layne Gulli told reporters. "You don't know how to avoid it. You don't know if you're next, or if your friend is next, or if it's an epidemic."

Fewer than 10 students and one member of staff were affected by the symptoms, but worried parents urged officials to close the school. Earlier this month, about 300 of the school's 1,200 students were absent. The outbreak mystified the community. "There's rumours it was carbon dioxide from the photography room," said student Joe Bradshaw. "We heard it was lead paint. Nobody knows." But the blogosphere went into speculative frenzy: drugs, chronic fatigue syndrome, swamp gas, raging teen hormones and an elaborate prank were all blamed.

On Monday, after weeks of tests, Virginia's Department of Health ruled out any environmental factors. "We believe the school is safe," said Dr Stephanie Harper. Instead, the department argued: "Stress in the group setting may have played a role in causing the symptoms experienced." It called the outbreak "sociogenic in nature".

Dr Jim Bolton, consultant psychiatrist at St Helier hospital in Surrey, said: "There's a strong possibility that it's a case of mass hysteria. It tends to occur in close communities, particularly schools, sometimes sports teams and convents. It occurs across all kinds of cultures, and is very common, particularly at times of stress. I remember when I was at school there would be fainting epidemics. Girls for weeks would be fainting left, right and centre. Generally what happens is one person experiences symptoms and then someone else with whom they're in close proximity experiences them too. And then it mushrooms."

Bloggers have been keen to compare the Virginia case with the Tanzanian laughing epidemic of 1962. At a mission-run boarding school near Lake Victoria, three girls started laughing. The symptoms spread, forcing the school to close, but that made matters worse: laughter attacks were reported at Nshamba, the home village of several of the girls. Before the epidemic abated two and a half years later, about 1,000 people in Tanganyika (as Tanzania was then known) and Uganda were afflicted.

At William Byrd high school they're still baffled. Dr Lorraine Lange, Roanoke County schools superintendent, said: "I don't think students here are differently stressed from any other high-school students."

Why that school rather than thousands of others should be susceptible to mass hysteria is, for the moment, unclear.