Last fall, when a dozen teenage girls in a single upstate New York high school developed a condition that looked like Tourette’s syndrome — complete with sudden verbal outbursts, uncontrollable arm motions and facial tics — it seemed likely that a chemical toxin or infectious agent was to blame. But none could be found.
More recently, three other students at Le Roy Junior Senior High School, including one boy, developed the same baffling condition, prompting parents to call in the big guns: environmental activist and heroine of a Julia Roberts movie, Erin Brockovich, who has just stepped in to investigate possible environmental causes.
Brockovich’s team said it would be six weeks before tests of groundwater samples near the school could identify contamination, but in the meantime, doctors have already diagnosed most of the girls with a disorder: mass psychogenic illness — otherwise known as conversion disorder or, to use an outmoded term, mass hysteria.
Mass psychogenic illness is thought to be triggered by stress or emotional distress, in response, for example, to reports of a chemical exposure, toxin or virus. The symptoms — which can and have included everything from uncontrollable dancing, unstoppable laughter and fainting spells to fits of meowing and penis shrinkage — spread through groups by way of humans’ often unconscious social mimicry of one another’s behavior.
(MORE: Exposure to Common Chemicals May Weaken Vaccine Response)
The condition may sound unlikely, but it is real, and it has in the past caused significant problems for emergency services. For example, after terrorists released toxic gas in the Tokyo subway system in 1995, commuters fell ill with mass dizziness and nausea. But doctors found that more than 70% of the 5,500 people who sought help at hospitals for gas-related symptoms turned out not to have been significantly exposed [PDF].
Similarly, strange smells in schools, businesses and factories have set off numerous outbreaks of fainting, nausea and cramps in the absence of actual chemical dangers, typically affecting only those who have seen other affected people or who believe the smell is dangerous. Recent decades have seen cases in Jordan, France and Colorado.
A 2011 study led by Joan Broderick of Stony Brook University in New York found that psychogenic symptoms can even be deliberately induced in normal, healthy adults. In the research, participants were given a pill and told that it was an experimental drug that had mild side effects and was being tested to increase effectiveness of flu treatment during a pandemic. Sixty-seven people participated in the study, which took place in a hospital.
Researchers divided the participants into three groups: one group received the pill (actually a placebo) in the presence of actors who also took it and displayed symptoms like nausea, headache and dizziness. A second group took the pill in the presence of actors faking symptoms and also watched a documentary about pandemic flu. A third group simply sat in the waiting room after taking the pill.
The participants who took the pill with the actors were 11 times more likely to show signs of illness than the control group — regardless of whether they watched the documentary. Some people developed symptoms that the actors had not even displayed. (They were all debriefed about the research afterward, and none objected to the earlier deception.)
Of course, labeling a condition as “psychogenic” or, worse, “hysteria” seems belittling and demeaning. Many people mistakenly believe that this means affected people are faking their symptoms and can control them. Despite the strides made by modern neuroscience, the stigma associated with conditions that are not physical in origin or “all in your head” still runs deep.
Not surprisingly, some parents and affected students in Le Roy, N.Y. — some who have had to drop out of school because of their condition — have objected to their diagnosis. One father told the Today show earlier this month, “Obviously we are all not just accepting that this is a stress thing … It’s heart wrenching. You fear your daughter’s not going to have a normal life.”
(MORE: How Childhood Trauma Can Cause Adult Obesity)
But stress is not just a mental phenomenon. Broderick explains that stress can actually change the body’s physiology. “Stress responses are not just psychological,” she says. “They also involve physiological responses [like] increased heart rate.”
In mass psychogenic illness, she says, “we believe it is the physiological response that individuals misinterpret as evidence of infection [or] contamination. This leads to fear and even more anxiety, creating a powerful experience of illness.”
The examples stretch back to the Middle Ages and early Renaissance, when outbreaks of twitching and tics led to witch hunts and persecution of those who were thought to have caused the problem. Convents held nuns who couldn’t stop meowing or shouting obscenities. Many towns reported incidents in which people literally danced themselves to death.
Such historical cases may be dismissed as exaggerated, fabricated or perhaps traced back to real pathogens, but similar incidents have been documented around the world far more recently. In 2007, a group of 14 Florida girls attending the same high school developed a disorder called stridor, when breathing causes a high-pitched wheezing sound like what would be caused by food or some other obstruction in the windpipe. No biological cause was found, and the girls improved after several months.
Also in 2007, an outbreak of twitching arms and legs spread through another group of high school girls (and one teacher) in Roanoke, Va. That episode also resolved itself with time.
In 2002, in Nigeria, a woman barely escaped being lynched by an angry crowd, after being accused of stealing a man’s penis. Koro, the belief that genitals have been snatched or have been shrunk back into the body by witchcraft, is widespread in Africa and some Asian cultures, and the condition often spreads after an initial case is reported.
Some psychiatrists have also suggested that sick building syndrome, in which building occupants develop health effects from spending time in the building, as well as multiple chemical sensitivity, a similar condition that is not linked to a specific place, are not caused by environmental issues but are also mass psychogenic illnesses. Another example may be Morgellon’s disease — a condition in which people report skin lesions and the feeling of bugs crawling under their skin: the Centers for Disease Control and Prevention recently determined the disease does not have an infectious origin.
(MORE: Morgellons Mystery: No Medical Explanation for ‘Crawling Skin’ Disease)
So how exactly does stress cause such a bizarre range of conditions, and why do symptoms spread from person to person? To answer the first question: the brain’s stress response systems affect every organ in the body. That is why, for example, early childhood stress can lead to increased risks for conditions as varied as heart disease, depression and diabetes later in life.
Further, these stress response systems are highly sensitive to social signals: social support is critical for stress relief, and social rejection or conflict is one of the greatest sources of stress. Our brains also naturally and unconsciously engage us in constant mimicry; for example, when we see someone smile, it automatically activates our own smile muscles slightly, even if we don’t actually reciprocate. In ordinary conversation, we adjust our speech and rhythms to those of around us, typically without conscious awareness. The brain even contains mirror neurons that fire not when we act ourselves but when we see someone else performing an action.
All these factors make us susceptible to fads, fashions and trends — as well as to cultural ideas about sickness and health. The research suggests that mass psychogenic illness tends to occur at times of significant social stress and that it is most likely to affect teenagers, particularly girls. That makes sense, since adolescence is a time of high stress, emotional chaos and heightened attention to social signaling, especially among girls. Other factors that increase susceptibility include being extroverted (and therefore more social) and having had greater exposure to traumatic experiences.
“They are really having symptoms,” says Broderick of affected people such as the Le Roy teens. “But rather than being due to a toxin, they are due to stress. They should be dealt with as we do any ill individual.”
Broderick explains that health officials determine that the cause in cases like that of Le Roy is psychogenic by ruling out environmental factors, including possible toxins, infectious disease, drugs, adverse vaccine reactions, food allergies and the like. “It is almost impossible to prove that a toxin is not present. Thus, it is converging evidence that is used to make conclusions,” Broderick says. “Of course, if the source of the investigation is not trusted, then that adds to the doubt. In fact, in situations where a government is not trusted by the population, it is more likely that psychogenic illness will persist as opposed to resolving quickly within a few days or weeks.”
Parents and students in Le Roy have expressed dissatisfaction with the environmental testing conducted by health officials. Similar to other cases of mass psychogenic illness, however, the disease detective work involved in Le Roy may be made easier by the fact that the patterns of spread of psychogenic disease do not match those of an infectious illness.
(MORE: The Upside of Being an Introvert [and Why Extroverts Are Overrated])
Psychogenic illnesses and the suffering they can cause are real and demand greater understanding, but health officials have to be careful to ensure that such conditions are not used to mask genuine environmental dangers or to dismiss related concerns.
Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.