A mystery illness health experts are describing as a type of "mass hysteria" has struck students at four schools in Bangladesh in the past week, forcing them to close temporarily.
The condition appears highly contagious -- as soon as one student becomes ill, others are immediately struck with similar symptoms, usually headaches, acute pain and even fainting, officials say.
Most of the victims are teenage girls, said Salahuddin Khan, chief medical officer of Jessore district, where the affected schools are.
"It's a peculiar disease. I've never seen anything like it," he told AFP.
"It started after a girl fainted at a school. Soon enough dozens of her friends complained of acute headaches, restlessness and body pains. They were all affected within minutes."
Speculation among local media about the origin of the baffling illness has been rife, with a top army doctor even telling state-owned broadcaster BTV that it was caused by poisonous gas "sabotage" against the impoverished country.
"So far, 81 students at three schools and a madrassa (religious school) have been struck by the ailment," said Khan.
"More than half of them fell unconscious and had to be hospitalised. We have shut down the schools and the madrassa temporarily."
He said a team from the country's Institute of Epidemiology, Disease Control and Research (IEDCR) visited the students and took blood samples.
The chief medical officer of central Narsingdi district said his region had also been hit.
Birendranath Sinha said 11 girls became ill on Monday after hearing one of their fellow students had died suddenly.
"There is no proper explanation for the disease. It's something mass psychogenic," he said, adding that hundreds of students also fell ill in schools in the district last year.
Work was also temporarily suspended at a garment factory in the southeastern port city of Chittagong last month after 700 workers complained of similar symptoms.
"It started after a worker said she saw a ghost-like object and within minutes the whole factory was in the grip of mass hysteria -- workers were falling unconscious, complaining of headaches, muscle twisting and breathlessness," said Mostaq Hossain, an expert who is spearheading IEDCR's research into the phenomenon.
The phenomenon is not new -- according to IEDCR statistics, about 2,000 pupils in Bangladesh have been affected by "mass hysteria" since it was first reported here in August 2005.
Last year, the government called an urgent summit involving all regional health officials to discuss the condition.
Health officials are calling for calm, saying there is no reason to be alarmed.
"We have identified it as 'mass sociogenic' disease. It is nothing but mass hysteria," Hossain said.
"Mostly teenage girls who are physically and mentally vulnerable are being affected by the disease. We've told health officials across the country not to panic. Schools have been told to improve the conditions for girls."
Hossain said the phenomenon was not a recognised psychiatric condition and generally affected groups, triggered by an unusual illness or shocking event experienced by one group member.
He also said there had been a snowball effect, with many girls who read about cases "feeling that they could be the victims of the same ailment."
He ruled out any poisonous gas theory.
"It was also common among English girls during the nineteenth-century Industrial Revolution in Britain," he said.