The schools fell like dominoes across Portugal in May 2006, one after another calling upon government officials with reports of dozens, then hundreds of students struck with rashes, dizziness and difficulty breathing, just as year-end exams approached. Was it a mysterious allergic reaction, a chemical spill, a virus? After digging deeper, medical practitioners came up with a new culprit: “Strawberries With Sugar,” or in Portuguese, “Morangos com Acucar.” No, not the food—the vector for this disease was a popular teen soap opera with a saccharine title. Just before the outbreak in the real schools, a similar, life-threatening illness had plagued the teenaged characters in their fictional school.

The Portuguese students weren’t suffering from a virus or allergies: they’d come down with mass psychogenic illness.

In a psychogenic illness, a psychological trigger—rather than a biological or environmental one—causes actual physical symptoms. As sociologist Robert Bartholomew explains: “Mass hysteria is the placebo effect in reverse. People can literally make themselves ill from nothing more than an idea." Bartholomew has studied mass hysteria extensively, and written about outbreaks around the world. “Parents and students fight the diagnosis as no one wants to accept that their kids were ‘hysterical,’” he said by email. “In reality, it’s a collective stress reaction and found in normal people.”

Also known as mass hysteria or conversion disorder, mass psychogenic illness can be divided into two main types: anxiety hysteria (triggered by extreme anxiety in a close-knit group and causing dizziness, headaches and fainting) and motor hysteria (which disproportionately affects girls and women, and results from long-term stress, causing twitching, shaking, facial tics and other muscular convulsions).

Or as science communicator Sian Hickson said at the Edinburgh International Science Festival during a discussion on the “Strawberries With Sugar” episode, this wasn’t simply a case of the 300 children making something up. “The pupils genuinely believed themselves to be ill and had rashes,” Hickson said.

The history of mass hysteria stretches back for centuries and impacts people of all cultures and regions of the world. Before the 20th century, mass psychogenic illness erupted in pockets throughout Europe, often in socially isolated convents where women were forced into highly stressful environments that included repetitive rituals like prayer, severe punishment for breaking rules and near-starvation diets. All of these conditions were enough to create the long-term stress necessary for motor hysteria. Between the 15th and 19th centuries, nuns were recorded bleating like sheep, barking like dogs and meowing. There have been some suggestions that similar types of stress were at play during the Salem witch trials, and that mass hysteria may have been a factor.

As the world transitioned through the Industrial Revolution, new sites emerged for these mass hysteria outbreaks: factories and boarding schools, which created the same kind of pressure-cooker situation as the convents. Then, starting in the 20th century, documented outbreaks began being triggered by concerns about environmental toxins in food, air and water, as well as fear of mysterious odors.

“Mass sociogenic illness [MSI] flourishes where the threat has a basis in reality. The 1995 terrorist attacks using sarin nerve gas on the Tokyo subway system… triggered a series of MSI episodes involving benign odors,” Bartholomew and psychiatrist Simon Wessely write in a paper on the history of mass hysteria.

It’s tempting to point to these outbreaks as quirky historical events with little bearing on society today. But modern psychogenic outbreaks can be extremely costly to hospitals and ultimately the U.S. healthcare system, since doctors can end up repeatedly test for an organic cause when one doesn't exist. And the rise of social media only seems to be making matters worse, because a trigger—other people experiencing a pyschogenic illness, a news article on an outbreak—can spread further and faster.

“We may be at a crossroads in the history of psychogenic illness, as the primary vector or agent of spread appears to be the internet and new technologies,” Bartholomew said by email. In his research, Bartholomew found only four cases of motor hysteria in Western schools throughout the 20th century, versus anxiety hysteria, which was far more common. But since 2002, he’s already recorded five outbreaks of motor hysteria. “These technologies have been evolving so rapidly, we have not had enough time to assess their impact.”

Neurologist E. Steve Roach, who worked on a case of motor hysteria in North Carolina that caused 10 teenage girls to experience seizure-like attacks, also thinks mass media, like television news coverage, can exacerbate the problem. Citing the 2012 case of motor hysteria in Le Roy, New York, in which young women experienced motor hysteria, he said, “It’s hard for me to believe that having it play out on national television is going to do anything other than make it more difficult to deal with.”

Roach added that the “Strawberries With Sugar” case is particularly interesting because it isn’t the media (news reports) exacerbating an existing case; the media (the television show) was the cause. In the age of Youtube, Twitter and online television, will the way media is consumed influence future outbreaks of mass psychogenic illnesses?

The teenage drama “Strawberries With Sugar” continued to draw viewers for several more seasons following the virus episode, and media reports suggest the afflicted students returned to school after the outbreak, but information about this incident is sparse and can only be found in the local press. Most cases of mass hysteria, including what happened in Portugal, remain unstudied by sociologists, neurologists and psychiatric professionals, according to Roach and Bartholomew.

The lack of studies on the MPI isn’t helped by stigma against being diagnosed with a psychological disease. “People who have these psychological conditions, the likelihood of that individual turning out to have a major disabling psychiatric disease is low,” Roach says. In some ways, this is good news; most patients with mass psychogenic illness recover. But it also makes the illness hard to predict, prevent, understand and explain.

“Just because your symptom—say blindness in the right eye—doesn’t correlate with inflammation with the right optic nerve, there’s still going to be a neurologic phenomenon that explains it,” Roach says. “There are plenty of things we can’t explain, but that doesn’t mean they don’t exist.” He experienced this in a particularly poignant way when diagnosing a young patient with psychologically mediated motor disabilities rather than a progressive and fatal degenerative disorder. For Roach, this was great news for the child and his family. But when he sat down to talk to the parents, they grew angry and left.

“If we can get past this aversion to psychological diagnoses I think that’ll help a lot,” Roach says.

As for Bartholomew, he wants to see the illness receive more attention everywhere, because it could strike anyone. “No one is immune from mass sociogenic illness because humans continually construct reality, and the perceived danger needs only to be plausible in order to gain acceptance within a particular group,” he wrote in his paper with Wessely. “As we enter the 21st century, epidemic hysteria will again mirror the times, likely thriving on the fear and uncertainty from terrorist threats and environmental concerns. What new forms it will take and when these changes will appear are beyond our capacity to predict.”