Throughout history there have been recorded instances of what used to be called 'mass hysteria'. This term is no longer in usage within the medical profession - instead it is called Mass Psychogenic Illness. Not sure that's a name that will catch on!
However, it is defined in the Medical Dictionary as follows:
1. Spontaneous, en masse development of identical physical or emotional symptoms among a group of individuals, as in a classroom of schoolchildren, with no known cause/pathogen.
2. A socially contagious frenzy of irrational behavior in a group of people as a reaction to an event.
It is has many different names and manifestations. In Nicaragua it is called 'grisi siknis' - Crazy Sickness, In Italy it was known as 'Tarantism' and was exhibited as frenzied dancing. The belief was that people had been infected by the bite of the Tarantula and dancing was the only cure, as in this way the venom was sweated out of the pores. It would usually start with one dancer and more and more would join in, some of whom believed that because they had touched a person who might have been bitten, they too we filled with the venom. That dance today is the Tarantella, and in fact the bite of the Tarantula in Italy (also known as the Wolf Spider) while it may hurt a little will not harm humans.
See Antonion Melechi's brilliant article on this phenomenon. In parts of South Africa outbreaks of mass hysteria are known as 'amafufunyana' (a wonderful word!) and are usually associated with girls' schools (as in fact most recent outbreaks of mass hysteria are).
Tanzanian Laughter Epidemic of 1962 is described here and it too started in a girls' school - affecting over 400 people and leading to school shut downs. Luckily no one died as a result - but in the Dancing Plague that struck Strasbourg in 1518 there were deaths. The phenomenon was described in the following way:
“Somewhere amid the narrow lanes, the congested wharves, the stables, workshops, forges and fairs of the medieval city of Strasbourg, Frau Troffea stepped outside and began to dance. No music was playing and she showed no signs of joy as her skirts flew up around her rapidly moving legs. To the consternation of her husband, she went on dancing throughout the day. And as the shadows lengthened and the sun set behind the city’s half-timbered houses, it became clear that Frau Trofea simply could not stop. Only after hours of crazed motion did she collapse from exhaustion. Bathed in sweat with muscles twitching, she finally sank into a brief sleep. Then a few hours later she resumed her solitary dance. Within days, more than thirty people had taken to the streets seized by the same urgent need to dance. By early August 1518, the epidemic was spreading at an alarming rate.”
The Dancing Plague of 1518 was a case of dancing mania that occurred in Strasbourg, France (then part of the Holy Roman Empire) in July 1518. Numerous people took to dancing for days without rest, and over the period of about one month, most of the people died from heart attack, stroke, or exhaustion. The authorities were convinced that the afflicted would only recover if they danced day and night. So town halls were set aside for them to dance in, musicians were hired to play pipes and drums to keep them moving, and professional dancers were paid to keep them on their feet. Within days those with weak hearts started to die. By the end of August 1518 about 400 people had experienced the madness.'
As the word hysteria originates with the greek world 'hystera' meaning uterus, hysteria was long used to describe behaviour associated particularly with females. It is no longer exclusively so, but it does seem as if outbreaks of mass hysteria now largely occur where groups of adolescent girls are gathered together. Between the 15th and 17th Centuries they were also widely reported from nunneries, and I think I would be right in saying that the entry age for nuns was a lot younger then, so it is conceivable that the same factors were at work as are found still today in boarding schools.
No real answers - but a fascinating phenomenon.