[2][3] It is the rapid spread of illness signs and symptoms affecting members of a cohesive group, originating from a nervous system disturbance involving excitation, loss, or alteration of function, whereby physical complaints that are exhibited unconsciously have no corresponding organic causes that are known.

[11] The DSM-IV-TR does not define a diagnosis for this condition but the text describing conversion disorder states that "In 'epidemic hysteria', shared symptoms develop in a circumscribed group of people following 'exposure' to a common precipitant."

[6] The hypothesis that those prone to extraversion or neuroticism, or those with low IQ scores, are more likely to be affected in an outbreak of hysterical epidemic has not been consistently supported by research.

[5] MPI outbreaks occurred in factories following the industrial revolution (1760–1840) in England, France, Germany, Italy, Russia,[5] the United States and Singapore.

In 1974, Stahl and Lebedun[19] described an outbreak of mass sociogenic illness in the data center of a university town in the United States Midwest.

[19] In 1962, the Tanganyika laughter epidemic was an outbreak of laughing attacks, rumored to have occurred in or near the village of Kanshasa on the western coast of Lake Victoria in what is now Tanzania, eventually affecting 14 different schools and over 1,000 people.

[24] Symptoms included headaches, dizziness, impeded respiration, muscle weakness, burning sensations, cramps, retrosternal/chest pain, dry mouth and nausea.

Rodovanovic stated that this reported instance of mass sociogenic illness was precipitated by the demonstrated volatile and culturally tense situation in the province.

The United Nations, World Health Organization and NATO's International Security Assistance Force carried out investigations of the incidents over multiple years, but never found any evidence of toxins or poisoning in the hundreds of blood, urine and water samples they tested.

[28][29] In 2011, a possible outbreak of mass psychogenic illness occurred at Le Roy Junior-Senior High School, in upstate New York, US, in which multiple students began having symptoms similar to Tourette syndrome.

Various health professionals ruled out such factors as Gardasil, drinking water contamination, illegal drugs, carbon monoxide poisoning and various other potential environmental or infectious causes, before diagnosing the students with a conversion disorder and mass psychogenic illness.

Robert Bartholomew, an American medical sociologist and author, said, "It is no coincidence that Kelantan, the most religiously conservative of all Malaysian states, is also the one most prone to outbreaks."

This view is supported by Afiq Noor, an academic, who argues that the stricter implementation of Islamic law in school in states such as Kelantan is linked to the outbreaks.

[31] Outbreaks of mass psychogenic illness "have been reported in Catholic convents and monasteries across Mexico, Italy and France, in schools in Kosovo and even among cheerleaders in a rural North Carolina town".

[35][36] In July 2022, reports of up to 15 girls showing unusual symptoms such as screaming, trembling, and banging their heads came up from a government school in Bageshwar, Uttarakhand, India.

On 29 April 2023, the Iranian Intelligence Ministry released the findings of a comprehensive investigation which concluded that the reported illnesses were not caused by any toxic substances.

[38][39] In October 2023, over 100 students from the St. Theresa's Eregi Girls’ High School in Musoli, Kenya were hospitalized due to rapid and involuntary arm and leg movement, sometimes accompanied by headaches and vertigo.

Far from faking it, “Under such prolonged stress, the nerves and neurons that send messages to the brain become disrupted, resulting in an array of neurological symptoms such as twitching, shaking, convulsions, and trance-like states.”[41] Bartholomew observes that school-stress borne illness such as occurred here have not been uncommon in Africa since the 1960s.

This may create overwhelming anxiety due to the students being taught one thing at home, such as ancestor worship, which is then forbidden at a Christian mythology based school.

This outbreak was surmised to be due to expectations of traditional arranged marriage, colliding with modernity's emphasis on romantic love that the students had observed in movies.

The difference between these two cases of mass psychogenic illness reinforces that each outbreak needs to be evaluated in the specific circumstances in which it occurred, as such instances are “never spontaneous reactions to stress per se; they are always couched in some unique context.”[43] In 2002, Bartholomew and Wessely stated that the "concern that after a chemical, biological or nuclear attack, public health facilities may be rapidly overwhelmed by the anxious and not just the medical and psychological casualties.

[5] Beginning in 2016, some staff stationed at the US embassy in Cuba reported medical symptoms that initially were attributed to "sonic attacks", and later to other unknown weaponry.

[44][45][46][47] Seven U.S. intelligence agencies headed by the CIA spent years reviewing thousands of possible cases of Havana syndrome and preparing a report.

Politico summarized the results by saying, "The finding undercuts a years-long narrative, propped up by more than a thousand reports from government employees, that a foreign adversary used pulsed electro-magnetic energy waves to sicken Americans.

The condition, known as resignation syndrome (Swedish: uppgivenhetssyndrom), is believed to only exist among the refugee population in Sweden, where it has been prevalent since the early part of the 21st century.

[49] In a 130-page report on the condition, commissioned by the government and published in 2006, a team of psychologists, political scientists and sociologists hypothesized that it was a culture-bound syndrome, a psychological illness endemic to a specific society.

[51][52] As evidenced by medical records, healthcare professionals were aware of this scam, and witnessed parents who actively refused aid for their children, but remained silent.