Researchers from the United Nations recently met in Cambodia to solve the mystery of why large numbers of female workers in that nation have succumbed to an unknown (and temporary) illness. But rather than a physical illness as the source, the scientists may want to look at the mind as the root cause.

(It's not that women are the only workers who are being affected, it's that the majority of workers in these factories are women.)

Workers in shoe and clothing factories have reported feeling fatigued, dizzy and nauseated. Most claimed that they felt faint, though none actually fainted. After some rest and medical attention, the women quickly recovered and went back to work; few if any reported lingering symptoms. So far no one has found any toxin or environmental contamination that could cause the symptoms.

Over a thousand workers have experienced similar episodes since June. The most likely explanation is mass hysteria, also called mass sociogenic illness. [Top 10 Mysterious Diseases]

Many people misunderstand the nature of mass hysteria and assume that victims are making up their symptoms. The complaints are real and verifiable; the victims are not imagining their problems. It is not a joke, nor a hoax. Nor is a diagnosis of mass hysteria merely a default explanation when investigators can't find a cause. In fact there are several fairly specific diagnostic criterion.

Mass hysteria often begins when individuals under stress convert that stress into physical ills. Co-workers, family and friends may also begin exhibiting the symptoms through contagion. Outbreaks are most common in closed social units (such as schools, hospitals and workplaces) and where afflicted individuals are under pressure and routine stress. Mass hysterias tend to afflict girls and women more often than boys and men, probably because the illness spreads through social ties, and females tend to have stronger social bonds than males.

Often the physical complaints are accompanied by reports of a strange smell, sound or lights. One of the most famous cases of mass hysteria occurred in 1997 Japan, when thousands of people claimed to suffer symptoms caused by flashing lights in the cartoon Pokémon. Only a small percentage of those afflicted actually had seizures; the rest were victims of mass hysteria.

Another famous case with elements of mass hysteria was the Orson Welles "War of the Worlds" radio broadcast in 1938, in which some people really believed that aliens were invading, and reported seeing attacking alien ships on the horizon. [7 Things That Create Convincing UFO Sightings]

There is no real treatment for mass hysteria (with the exception of attention from doctors or other authorities); the episodes tend to run their course and fade away almost as quickly as they started.

The Cambodian factories — full of women, chemicals, smells, stress and boredom — are ideal environments for the development of mass hysteria. Denial is typical in these cases; victims usually reject the diagnosis and remain convinced that some unknown agent is causing their discomfort. Unless investigators find another cause for the mystery illness, it will likely be traced to mass hysteria.

Benjamin Radford is deputy editor of Skeptical Inquirer science magazine and co-author of the upcoming book The Martians Have Landed: A History of Media-Driven Panics and Hoaxes. His research into the Pokemon mass hysteria was published in the Southern Medical Journal in 2001. His Web site is www.BenjaminRadford.com.