Collective hysteria can spread when a fear exists of exposure to a disease, combined with a contained, stressful environment.

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How much power do our minds have over our physical health? Is health simply a biological concept, or is there more to it? A recent story from Mexico may provide some clues.

In 2006, a mysterious illness began to affect girls at a boarding school in Chalco, Mexico, near Mexico City. The school, which is run by Roman Catholic nuns, is one of 10 in Asia and Latin America operated by a charity called World Villages for Children in Asia. The girls, ages 12 to 17, showed strange symptoms: difficulty walking, fever and nausea. After the girls returned from a 10-day Christmas break, the illness spread. Eventually 600 out of the 3,600 girls at the school showed symptoms. Still, no one could figure out what was making the girls sick, and public health officials were called in.

After conducting numerous tests, surveying the facilities and interviewing some of the afflicted girls, doctors have decided that a psychological disorder is responsible. Its official name is mass psychogenic disorder, also called collective hysteria, mass psychosomatic reaction or mass hysteria.

Mass psychogenic disorder is a rare -- but not unheard of -- phenomenon. The disorder is usually characterized by the mysterious spread of a variety of symptoms without a discernible cause. It frequently occurs in isolated communities. Teenagers and girls are also frequent victims. Collective hysteria can spread when a fear exists of exposure to a disease, combined with a contained, stressful environment.

Dr. Victor Manuel Torres Meza, director of epidemiology for the Mexico State Health Department, told the New York Times that there were 80 documented cases of mass psychogenic disorder around the world. In the case of the girls at the Mexican boarding school, they live in a highly structured environment, following a disciplined, regimented routine. Correspondence and interaction with parents is sparse -- children see their parents no more than three times a year. Between visits, letters are permitted; however, the girls are not allowed to call home. An environment with that combination of stressors likely contributed to the illness’ spread. The school eventually allowed parents to take their children home, and those who were sick recovered quickly.

In hopes of finding the trigger and learning more about this particular outbreak of mass psychogenic disorder, 20 doctors and psychologists have begun interviewing the girls who are currently or have been sick.