McEvedy first became aware of hysteria-triggered epidemics while working at the Royal Free Hospital in London in 1955. During that year, he saw an epidemic sweep through the nursing ranks of the hospital, altogether affecting 300 nurses, 200 of whom were confined to bed. Doctors carried out a thorough analysis, but were unable to discover any causative organism. In the end, the epidemic was blamed on a benign myalgic (muscle-affecting) form of encephalomyelitis, now better known as ME.
There the matter remained until McEvedy prepared a paper in 1970, which established that the epidemic was caused by hysteria that, in turn, was triggered by a fear of polio. Polio had been prevalent at the time of the epidemic, and it's possible that the nurses felt especially exposed as they were in regular contact with its victims.
In one report, McEvedy identified 15 similar cases, including eight arising in hospitals and seven among the general population (BMJ, 1970; 1: 7-11, 11-5).
In an earlier paper, he had also identified other hysteria-based epidemics that had occurred at several girls' schools. At one such institution, a small epidemic started after a number of girls from one classroom complained of abdominal pain and vomiting, and were taken to hospital and kept under observation.
The next day, what was described as an 'explosive epidemic' started during an assembly and affected most of the school. The main symptoms were feeling faint and 'peculiar'. The epidemic lasted nine days, although there were fewer cases on days four and five, which happened to be the weekend! As with the Royal Free outbreak, no causative agent was ever found (BMJ, 1966; 2: 1300-2).
McEvedy discovered that most victims of hysteria-based epidemics complained of flu-like symptoms, including depression, fatigue and vague neurological effects. Most of the girls also suffered more serious reactions such as paralysis and sensory disturbances. In all, the hysteria-based epidemics affected about 10 per cent of all women in a community group, such as a hospital, and just 2 per cent of men.