It started about three weeks ago, when about 20 children from the Elsa Perea Flores School in Peru became violently ill.
They displayed the same terrifying symptoms; muscular convulsions - some developing into full blown seizures - fainting, vomiting, delirium and frothing at the mouth.
The children, all aged between 11 and 14, shared another unsettling trait - a shared hallucination involving being chased by a "tall man in black with a beard" who was trying to kill them.
Since then, between 80 and 100 children from the school have been affected by what education officials are calling a mass case of "contagious demonic possession or interference".
Video footage recorded at the school shows screaming children being taken by trucks to nearby hospitals. Other children are seen lying on school desks surrounded by classmates and teachers who switch between comforting and restraining them as they wait for medical help to arrive.
"It's disturbing for me to think about it. It's as if someone kept on chasing me from behind," one affected student, 12, who has since recovered, told reporters.
"It was a tall man all dressed in black and with a big beard and it felt like he was trying to strangle me. My friends say I was screaming desperately, but I don't remember much."
A 13-year-old girl, who also wanted to remain anonymous, citing fears of being laughed at or attacked, told TV Peruana of several children from different classrooms fainting at the same time.
"I got nauseous and started vomiting," she said. "I heard voices. A man in black chased me and wanted to touch me."
Another schoolgirl said she had trouble breathing and was desperately holding her neck as if someone was strangling her. According to friends who say they witnessed the event, she kept screaming: "Take it out".
Initial reports suggested the children had been playing with a ouija board in an effort to call up spirits said to haunt the school, which is rumoured to have been built on land once used as a mass grave by the Mafia.
School authorities have called in a string of doctors, holy men and even exorcists since the outbreak began. Several masses have been carried out by Catholic priests at the school at the request of parents but so far nothing seems to have worked.
"We don't understand how this has kept on going on", Dr Antony Choy told national network Panamericana TV. "We know it started on April 29 and now it is still happening. Now there are more than 80 pupils (still affected)."
Elsa de Pizango, whose 12-year-old daughter was one of those who became "possessed" told reporters: "She fainted in school. They didn't say anything at the hospital. She just fainted. She keeps on spitting froth from her mouth."