Epidemiological enquiries into a school outbreak of an unusual illness

A total of 65 students and one female teacher were afflicted with an unusual illness following alleged inhalation of a 'gas' in the school. The main symptoms were dizziness, chills, nausea, headache, difficulty in breathing and faintness. Initial investigations revealed elevated carboxyhaemoglobin levels (greater than 5%) of 16 hospitalized students. However, no sources of carbon monoxide emission could be detected despite extensive environmental investigations. The outbreak was characterized by the following features: most victims were adolescent girls; there was a high proportion of recurrences; the illness was self-limiting with inconsistent clinical and epidemiological findings; its mode of transmission was associated with antecedent visual observation or verbal information, and it died down immediately after confidence had been restored. A diagnosis of mass psychogenic illness was made. The high-risk students were Malay girls who were from the normal class (for slow learners), engaged in part-time employment and easily influenced by what they had heard or seen.