They're found in many faiths, but are such experiences spiritual or an outlet for extreme stress?
(Reuters) Lina, a former worker at a cigarette factory in Indonesia, says she was 17 the first time she was possessed by an evil spirit.
"My older sister went down first. She was screaming and her body went rigid and she couldn't move. Then the spirit came into my body too," said Lina, who like many Indonesians has one name.
Reports of schoolchildren, young women and factory workers going into mass trances or speaking in tongues are common across Indonesia's vast archipelago of 226 million people.
The phenomenon may provide an outlet for stress, some experts say. In many cultures, it is part of a spiritual experience, whether in the voodoo trances of Haiti, the mass hysteria of Europe's witch trials, or Christian exorcisms.
Recently, local news station Metro TV broadcast footage of 11 students and five teachers in a mass trance at a school in the central island of Sumbawa.
"Every society has some kind of culturally appropriate place for trance experiences, usually in religious settings," said Tanya Luhrmann, a Stanford University anthropologist who studies witchcraft and evangelical Christianity, where group faintings are common.
"There appears to be a contagion element to trance, but it really requires some kind of willingness on the part of the individual," she said in an emailed reply to questions, adding that this was the case even if it seemed unconscious.
Religion, education and development have done little to budge widespread acceptance of the supernatural among Indonesia's ethnic and religious groups.
"In Indonesia, trance is tied up with culture," said Lidia Laksana Hidajat, research coordinator in the psychology faculty of Jakarta's Atma Jaya University.
Indonesian media reported a group trance among workers at Bentoel's cigarette factory in Malang, Java, in March 2006. Hidajat interviewed 30 of the affected women for her research.
"They told me that when it happened, they were sitting in a very long hall, working together in rows, rolling the cigarettes by hand," she said. "They were working in silence. That's one of the requirements of a trance to happen - it's usually quiet and when they are engaged in monotonous activity."
Suddenly, one of the workers started screaming and her body went stiff. The one next to her went stiff too. Others tried to help but soon they started too.
A local Muslim leader was summoned, but his prayers had no effect. Eventually, the exhausted women fell asleep and when they awoke they remembered nothing.
Hidajat concluded that the mass trance had more to do with exhaustion and stress: "Often, they are people who are very religious or under pressure."
Eko Susanto Marsoeki, director of Malang's Lawang Psychiatric Hospital, cited also overwork.
"Usually this happens to people who had problems in their childhood and to people who are working too hard. It's a form of dissociation, a kind of hysteria," he said. "They can't protest, but they can protest via a mass trance. So often it is a form of protest that will not be dealt with too harshly," he said.