Mass hysteria is defined as a condition in which a large group of people exhibit the same state of violent mental agitation.
The definition of mass hysteria is clearly seen through an historical event that occurred in Blackburn, England in 1965. On Thursday, October 7th 1965, several girls in a girl's secondary school had complained about feeling dizzy and some had fainted. By midday, several other (about 85) other girls had fainted. The girls were taken to the hospital in an ambulance and the school was closed for the week. So does this mean a huge group of people all of sudden came down with same epidemic at once? Our initial thoughts would be that whatever this disease is, it must be very contagious and strong. However, throughout several tests, such as white blood cell count (check for immune response), neurotoxic reagents that could have been in food, and such, everything seemed to be normal and there seemed to be no biological link to the cause of this incident. The girls who were affected scored higher for extroversion and neuroticism in the Eysenick Personality Inventory, indicating their vulnerability to 'follow the crowd'. A possible hypothesis proposed by the paper is that the fear stemmed from the polio epidemic the town suffered earlier in the year. This case is a great and interesting example of fear as the sole reason for the dizziness and fainting in students that led to mass hysteria.
Moss, P. D., and C. P. McEvedy. "An Epidemic of Overbreathing among Schoolgirls." Bmj 2.5525 (1966): 1295-300. Print.