The girl had been under a lot of stress when her leg inexplicably started to shake. The twitching then spread to a group of girls in her high school, sparking widespread anxiety.

It was 1939, in Bellevue, La.

Almost seven decades later, something similar is happening at William Byrd High School in Roanoke County.

Today, scientists say the Louisiana twitching was a case of "mass psychogenic illness," a psychological condition caused by anxiety that can produce seemingly inexplicable symptoms.

Robert Bartholomew, who has studied the phenomenon extensively, said he has no doubt that what caused the Louisiana symptoms is also causing the symptoms at William Byrd.

"The events at William Byrd High have all the hallmarks of a collective stress reaction," he wrote in an e-mail.

Young women who are living with a great deal of pent-up stress are more susceptible to psychogenic illnesses, according to Bartholomew, a sociologist in Australia who has been following the case at William Byrd via the Internet.

"The symptoms aren't just 'all in their heads.' They're real, though the cause is psychological," he wrote.

School officials say they have heard from roughly 10 people at William Byrd complaining of the twitching symptoms, including one female teacher. Students and parents have said that all the students affected are also female.

School and health officials have so far not revealed the cause of the symptoms, which the Virginia Department of Health described as "involuntary movements in the extremities." Tests done on the school's air, water and surfaces have turned up nothing unusual.

Neither the school system nor the health department would comment on the possibility that the symptoms could be signs of a psychogenic condition. The health department with the help of the Centers for Disease Control and Prevention has been investigating the symptoms and is expected to discuss its findings this week.

The mysterious outbreak has troubled parents, hundreds of whom kept their children home from school last week even though school officials said the building was safe.

The affected teacher was seen by doctors at the University of Virginia Health System, who cleared her to go back to work last week.

Dr. James Stone, a family doctor in Wytheville, said the description of the symptoms led him to believe the school system is dealing with a psychogenic condition.

"These symptoms are so unusual and they just don't fit in the right box. You like to have things that you can put a ribbon around and say this is it," he said, adding that he did not mean to trivialize the parents' concern.

"If I'm wrong I'll be the first one to eat a bowl of crow," he said.

There are two types of mass psychogenic illnesses, Bartholomew said. The first causes hyperventilating, dizziness and headaches, stemming from an unfamiliar odor. The second is known as "mass motor anxiety" and produces "outbreaks of twitching, shaking and trembling," he wrote.

It's been a busy few months for the Roanoke County Public Schools. School officials had to deal with loose asbestos at Northside High School, confirmed staph infections around the region and repeated warnings about Methicillin-resistant Staphylococcus aureus, also known as MRSA, after a Bedford County mother said her son died of the disease last month.

On top of that, school districts around the area have been on edge after school shootings, such as the one in April at Virginia Tech, dominated the news.

Still, some William Byrd parents say they don't believe anxiety is behind the symptoms.

"I don't see where it's been anything going on there that would be more stressful than any other time of the year or any other year, so I don't think that stress is the cause of it," said Sherry Beckner, the mother of a sophomore.

"I know all the kids but one," said Angela Bradshaw, who has emerged as a spokeswoman for the school's parents. "They're very happy children."

Bartholomew said psychogenic episodes were common in more repressed medieval societies but have become rarer in the Western world as people now have more ways to release pent-up stress. Still, he wrote, "outbreaks are much more common than are reported in the media."

In 2004, chorus members at Starpoint High School near Buffalo, N.Y., reported feeling dizzy, headaches and nausea. Extensive tests on both the students and the building found nothing.

The following year, a "mysterious odor" at West Cedar Elementary School in Waverly, Iowa, was blamed for fatigue, headaches, itchy eyes and dry throats among roughly 10 percent of the student body, according to an article co-written by Bartholomew earlier this year. Health officials and the Environmental Protection Agency found nothing wrong in the school.

Closer to home, a mysterious "mad gasser" sparked panic in Botetourt and Roanoke counties in 1933 and 1934. Residents claimed that someone had sprayed their houses with some poisonous gas that was making them sick. An analysis of the gas found it to be similar to common insecticides, leading to a Roanoke Times 1934 headline: "Sample of 'Gas' Is Found To Be Harmless To Humans."

Staff researcher Belinda Harris contributed to this report.