It started with nothing more dramatic than a funny smell.

A teacher noticed it first in a school kitchen. She felt sick to her stomach. Several of her students felt strange. Soon, by the dozens, students throughout the school complained of nausea, dizziness, headaches and drowsiness.

Before it was over, more than 170 students, teachers and others sought emergency treatment during the November 1998 outbreak at Warren County High School in McMinnville, Tenn. The school of 2,000 students was closed for more than two weeks. Nearly $100,000 was spent on emergency care alone.

A raft of government investigators studied the school grounds and analyzed blood samples. They examined puddles and grease traps and even checked an air space above the foundation. They looked for viruses, germs, pesticides, herbicides, poisons _ anything that could conceivably make so many people ill so quickly.

They found ... nothing.

In an article published in Thursday's New England Journal of Medicine, they blame the outbreak on mass hysteria _ real symptoms set off by spreading anxiety.

The investigators say such outbreaks are much more common than recognized at schools and in other close-knit communities. And they say cases of mass hysteria _ now called ``psychogenic illness'' _ will probably become more frequent and more severe with rising public worry over biological terrorism.

New technology may also be promoting new forms of hysteria. Dr. Ron House, a University of Toronto researcher who has studied mass hysteria in Canada, said Internet messages about real or imagined symptoms of conditions like chemical sensitivity now appear to be touching off such outbreaks in people at great distances from one another.

The phenomenon, though, has long historical roots. Hundreds of outbreaks of mass hysteria have been documented over the past century, often among schoolchildren, soldiers and factory workers. Some historians attribute a feverish medieval mania of jerky movements, known as Saint Vitus' Dance and once blamed on demons, largely to mass hysteria.

``In a previous era, spirits and demons oppressed us. Although they have been replaced by contemporary concerns about invisible viruses, chemicals and toxins, the mechanisms of contagious fear remain the same,'' said Dr. Simon Wessely of Guy's School of Medicine in London.

The Tennessee investigators are emphatic in saying that no virus, chemical or toxin spread the illness through the school.

``We did the most thorough environmental and epidemiological study that was humanly possible,'' said Dr. Timothy Jones, a federal epidemiologist now at the Tennessee Department of Health and lead author of the report.

He said it is still possible the outbreak was touched off by a real chemical smell, though such a chemical was never discovered. But he said mass hysteria soon took over and brought on the vast share of illness.

In fact, the investigators found some classic hallmarks of contagious anxiety: Victims were about three more times more likely than others to have seen another sick person or to know that a classmate was ill. Sixty-nine percent were women, compared with 52 percent of the school population.

Psychogenic illness is imperfectly understood. Females have long been thought prone to it, though some experts now dismiss that as the bias of male researchers. Others suggest that working women feel less control over their environment than men _ another classic factor in hysteria. Outbreaks, often worsened by news reports, are also thought to be more likely in groups under great psychological or physical stress.

The impact was so profound at the Tennessee school that it remains a sensitive topic for Principal George Bolding.

He said he smelled an odor himself and came down with a headache. But he agrees that the outbreak later did turn into hysteria. ``Kids are unreal about copying,'' he said. ``They see someone get sick, and they get sick.''

Researchers say outbreaks can sometimes be stemmed with fast reassurances. But people often resist the mass-hysteria explanation, because they find it embarrassing. Also, there are real environmental and biological threats that often can't be ruled out without time-consuming tests.