Experts have struggled to explain how 40 children fell ill in one school after an Armistice Day service in which four pupils fainted.
Toxic exposure and psychological triggering have been suggested following the spate of illness on Wednesday at Outwood Academy School in Ripon, North Yorkshire.
Prof David Coggon, Professor of Occupational and Environmental Medicine at the University of Southampton, said: “Fortunately, incidents like this are rare, so we cannot say statistically what is the most likely cause from past experience.
“However the possibilities include an unusual acute toxic exposure, and a psychologically mediated response, perhaps triggered by one child fainting in hot and stuffy conditions, and growing anxiety as others also became ill.
“To distinguish between these possibilities, health professionals will first need to make clinical assessments of the individual children who have become ill, and check the school for any possible sources of unusual hazardous exposures.
“It may also help to compare the characteristics of children who did and did not become ill – e.g. were those affected all in the same room, and were they all near to each other?”
Hysteria is among of the causes suggested by observers.
Professor Christopher C French, who is head of the Anomalistic Psychology Research Unit at Goldsmiths University’s Department of Psychology, said: “Outbreaks of mass hysteria have been recorded on many previous occasions. In general, they are simply due to the power of suggestion. In cases such as these, once one or two children have fainted – in this case, the hall was quite warm – a degree of panic sets in.
“The symptoms of panic then cause more children to feel dizzy and faint and a ripple effect can ensue. Typical symptoms include dizziness, nausea and so on – all of which can easily be induced in suggestible individuals.
“Previous similar examples include the so-called ’June bug epidemic’ which occurred in 1962 in a textile factory in USA.”
The June bug epidemic saw 62 workers fall ill, complaining of numbness, nausea, dizziness and vomiting, after rumours spread of a bug whose bite would cause the symptoms.
US public health authorities concluded that the June bug epidemic was a case of mass hysteria, exacerbated by stress in the workplace.