Science has proven that medical illnesses can transfer between people by way of viruses. There's nothing mysterious about influenza and head colds spreading from close contact with sick people. Harder to explain, however, are incidents in which symptoms appear out of nowhere or seemingly impossible events are experienced by a large population. Such episodes fall under the blanket psychological term of "mass hysteria," and here are a handful of examples.

1. Salem Witch Trials (1692–93)

This is one of the best-known incidents of mass hysteria. It began when two young girls of the small town of Salem Village began to experience seizures that were not explained by contemporary medical science. After their seizures, the girls proclaimed that they were being assaulted by supernatural entities conjured up by local women.

Soon more girls were being afflicted and more townspeople (mostly females) were being accused. Trials were promptly enacted and those women who did not confess were sentenced to death. Ironically, those who falsely confessed did not face execution.

More than 20 people were executed and more than 100 were jailed before common sense was restored. The girls' seizures ended and the trials ceased. Later medical scientists would lay the blame on everything from stress caused by ongoing warfare with American Indians to fungus on bread products.

2. The Miracle of the Sun (Oct. 13, 1917)

People gather to witness the 'Miracle of the Sun' on Oct. 13, 1917 in Fatima, Portugal.People gather to witness the 'Miracle of the Sun' on Oct. 13, 1917 in Fatima, Portugal. (Photo: Illustracao Portugueza/Wikimedia Commons)

Including this occurrence on the list of mass hysteria is controversial, as many people believe in its authenticity. But some researchers have concluded that this was indeed a case of mass hysteria, and for that reason, it's presented here.

In May 1917, in the environs of Fatima, Portugal, three young children claimed to have received an apparition by the Virgin Mary. There would be several subsequent supernatural visitations before a complex and hotly debated incident that took place on Oct. 13, 1917.

Mary had reportedly told the children that on that day she would appear to them for the last time and would create a miracle so astounding that everyone would believe in the authenticity of her appearances. What occurred is still up for dispute.

A crowd of thousands gathered to witness the promised miracle. The day featured a steady rain until the sun broke through the clouds. Many of the witnesses said that the sun's light appeared abnormal and that the sun changed colors and spun around in the sky. A Portuguese newspaper reported " ... the sun trembled, made sudden incredible movements outside all natural laws — and the sun 'danced.'" Others said the sun appeared to drop out of the sky and hurtle towards Earth.

Many people claimed to have witnessed these phenomena, but many other people said they only saw some of these unnatural events and some claimed not to have seen anything unusual at all. Because there were no reports of unanimous agreement as to the solar events of that day, later researchers have suggested mass hysteria or temporary eye abnormalities caused by looking directly at the sun as the cause of the episode.

3. Halifax Slasher (1938)

A more personal case of mass hysteria occurred in the town of Halifax, England, a town in West Yorkshire. Beginning in November 1938, reports were made to the police regarding a man attacking people, mostly women, with a mallet or knife. The number of alleged attacks grew to a size unmanageable by the local law authorities and Scotland Yard was called in to help the Halifax police.

Those police soon became suspicious of what looked like self-inflected wounds on the "victims." It didn't take long for several of those claiming to have been attacked to admit that they had, indeed, faked their encounters with the phantom perpetrator. In early December a local newspaper proclaimed: "The theory that a half-crazed, wild-eyed man has been wandering around, attacking helpless women in dark streets, is exploded ... There never was, nor is there likely to be, any real danger to the general public." The authorities concluded that all of the alleged victims had made up their claims in a bid to garner attention and pity — although they allowed that many of them may have believed they had been attacked.

4. Tanzania laughter epidemic (1962)

In late January of 1962, at an all-girls school in Kashasha, Tanzania, three girls began to laugh uncontrollably. They didn’t stop, even when disciplined by their teachers. Bizarrely, the laughter spread until more than 90 students were laughing for no apparent reason. The laughing fits allegedly lasted anywhere from a few hours to more than 15 days. Officials tried to find a cause but were unsuccessful. The epidemic laughter led to the closure of schools as the laughing spread to other villages nearby. Its cause remains unknown. The laughter episodes continued on and off for about one year, and then mysteriously ceased.

5. Sri Lanka (2012)

A more recent example of mass hysteria also involved schools. In mid-November of 2012, strange flu-like symptoms struck a school in the country of Sri Lanka. In addition to intestinal problems and fits of coughing, some children (and a handful of teachers) experienced rashes and headaches. In the small town of Gampola alone, more than 1,000 people were admitted to local hospitals. The area experienced widespread panic as the mysterious illness seemed to spread to other schools and centers of population. No medical cause could be found and the "illness" vanished as quickly as it had appeared. Not finding any other explanation, medical professionals concluded that those afflicted had been the victims of some type of mass hysteria strong enough to produce observable physical symptoms.

There are numerous other examples of mass hysteria featuring everything from the dubious 1789 series of attacks in London by a man with knives on his knees to the 1983 multiple episodes of Palestinian girls and young women suddenly losing consciousness for no discernible reason.

The final analysis of these episodes seems to reinforce the medical belief that the mind is a complex and powerful thing that can be affected by outside forces — both real and imagined.