School and health officials in Springfield last week initially assumed carbon monoxide was the likely reason a number of students suddenly fell ill, prompting an evacuation of the K-12 building.
But investigators found no trace of carbon monoxide at the southwest Minnesota school.
So what was it? There's no way to know for sure, officials with the Minnesota Department of Health say. But after a battery of air-quality measurements and a review of patient tests, officials say a process of "diagnosis by exclusion" has led them to conclude that it was likely a case of "mass psychogenic illness," the Mankato Free Press reports.
Mass psychogenic illness occurs when one person gets sick, and it prompts others in the group to get sick because they believe the same cause – such as carbon monoxide – is making them ill.
The physical symptoms are real – people genuinely feel ill – but there is no real cause that can be found.
The incident at the Springfield school Feb. 6 started with as many as 10 students getting sick in the auditorium during a choir rehearsal, and the illness seemed to be spreading. The school's 600 students were evacuated and 11 were initially taken to the local Mayo health facility.
By the end of the day, 30 people from the school had been treated there. Classes were canceled the next day as investigators continued to probe for a cause.
Such cases are not unheard of. A dozen teenage girls at an upstate New York high school in 2011 developed tic-like symptoms in the fall of 2011, and doctors diagnosed it as a case of mass psychogenic illness.
Joan Broderick, an associate professor of psychiatry at Stony Brook University who has conducted research on psychogenic illness and treated patients suffering from it, told Time magazine that the media play a prominent role in the phenomenon.
Investigators also ultimately concluded it was mass psychogenic illness that caused students to get sick at a high school in Tennessee in 1998, the National Institutes of Health reported.
After the incident, Timothy F. Jones, a doctor with the Tennessee Department of Health, wrote a paper urging other physicians to consider mass psychogenic illness as a possible cause in similar situations.
"Symptoms often follow an environmental trigger or illness in an index case," he wrote. "They can spread rapidly by apparent visual transmission, may be aggravated by a prominent emergency or media response, and frequently resolve after patients are separated from each other and removed from the environment in which the outbreak began."
Jones noted that mass psychogenic illness, also known as mass hysteria, has been described for more than 600 years in various cultures and settings, but is rarely addressed in medical training.