Hundreds of Afghan schoolchildren have been admitted to hospital in the past six weeks after falling victim to what is claimed to be six separate poison attacks.
Three occurred in northern Takhar province in the past week, affecting more than 300 girls.
Some government and police officials have blamed the Taleban, whose hostility to girls' education during its hardline rule in the 1990s is well documented.
Others have blamed the "enemies of Afghanistan" and hinted at the involvement of Pakistan and Iran.
But tests by the International Security Assistance Force (Isaf) and the government have not found any toxic substances. One international expert says the scares have all the hallmarks of mass hysteria.
In the most recent attack 170 girls in Takhar's provincial capital, Taloqan, were taken to hospital after falling ill and losing consciousness.
Pupils blamed poisonous gas, claiming to have smelled a noxious odour on entering their classroom at Ahan Dara Girlsi High School.
Students at Bibi Haji school also blamed toxic gas for poisoning them in two attacks on May 23 and 27.
Girls at another school in Takhar became ill in April and said the drinking water in their well had been deliberately contaminated.
More than 200 boys at a school in eastern Khost province also fell sick in mid-May, as did 100 girls in northern Balkh province on May 9. Their school said its well had been poisoned.
In all cases, most pupils who were admitted to hospital were released on the same day and no long-term damage was done.
Gul Agha Ahmadi, a media adviser at the Ministry of Education in Kabul, said that officials were awaiting test results from the most recent scares but that results from tests done after the incidents in April and early May had not found harmful substances.
Isaf tests into the Khost incident also showed no harmful substances present.
Mr Ahmadi said mass hysteria could not be ruled out, because Afghan people live in constant fear of insurgent attacks and could easily imagine terrorists poisoning their drinking water.
Robert Bartholomew, a prominent sociologist, also told the AFP news agency that the poisoning scare had "the tell-tale signs" of mass hysteria.
He said, "the preponderance of schoolgirls; the absence of a toxic agent; transient, benign symptoms; rapid onset and recovery; plausible rumours; the presence of a strange odour; and anxiety generated from a wartime backdrop" all pointed to mass hysteria.
And the Taleban have denied involvement, saying any of their members committing such actions would be punished.