1970 London outbreak
1970 London outbreak: In 1970-1971, at least 145 cases of epidemic neuromyasthenia (ENM) were observed in the staff of the Hospital for Sick Children, Great Ormond Street, London, England, although there may have been additional undiagnosed cases. The majority of those affected were nurses. The chief medical investigators, MJ Dillon, WC Marshall, JA Dudgeon, and AJ Steigman, believed this outbreak was similar to outbreaks of unknown etiology in the US, Denmark, Australia, South Africa, Germany, Greece, and the UK which were described by many names, such as, poliomyelitis-like illness, Iceland Disease, Akureyi disease, Royal Free disease, benign mayalgic encephalomyelitis, and epidemic neuromyasthenia.
The outbreak occurred in two waves. The first wave, between mid-August and the end of September 1970, had thirty-three cases. Most were student nurses with one case being a nun who taught the students. The second wave ended towards the end of January 1971 and affected all levels of hospital staff, including administrative and domestic staff. A few sporadic cases developed between February and June 1971.
The breakdown of who became ill was: 103 student nurses, 21 senior nurses, 6 doctors and 15 hospital staff. The vast majority were female, but four males (two doctors and two administrators), also, became ill. No patients were affected. 
Symptomatology in order of prevalence was headache, sore throat, nausea, back pain, malaise, vomiting, neck pains, tiredness, limb pains, depression, dizziness, sore eyes, cough, coryza, chest pain, abdominal pain, photophobia, diarrhea, earache, laryngitis, paresthesia, faintness, jaw pain, bladder symptoms, anorexia, subjective limb weakness, blurred vision, diplopia, and painful joints. A striking symptom reported was rapid fatigability with exercise and relapses over a prolonged period sometimes lasting several years. Duration of the illness varied greatly and was hard to determine because of relapses. Even those who recovered from the illness never fully regained their pre-illness level of health.
Invasive testing was kept to a minimum so as to not create an atmosphere of anxiety in a vulnerable population. Serological tests were done for viruses, such as cytomegalovirus, adenovirus, herpes simplex, influenza A and B, parainfluenza 1 and 3, mumps, and Epstein-Barr virus;electron and immune electon microscopy; cell cultures were inoculated into lab animals; hematological, biochemical, bacteriological tests; cerebrospinal fluid examination; immunoglogulin, interferon, phyohemagglutin stimulation of peripheral lymphphocyctes, and lymphocyte cultures testing was done. Laboratory and other testing was unable to uncover an infectious agent, although the investigators believed the pattern of the outbreak pointed to an infective etiology.
Mass hysteria was considered by several physicians not directly involve in the investigation or treatment of the outbreak. In 1974, in a letter to the British Medical Journal, the chief investigators defended their lack to psychiatric intervention: "we did not feel sufficient psychiatric symptomatology to justify psychiatric referral." They went on to explain that morale was good and no "belle indifference," that is, hysteria or conversion disorder, was observed.
In 1994, Nathaniel C. Briggs and Paul H. Levine, from the Viral Epidemiology Branch, Epidemiology and Biostatistics Program of the National Institutes of Health, wrote a review comparing twelve outbreaks referred to as chronic fatigue syndrome, epidemic neuromyasthenia, and myalgic encephalomyelitis. They grouped the outbreaks into four levels of increasing neurological involvement, ranked I-IV. The 1970 London outbreak was rated as level I, which meant that they found minimal neuropsychological changes. Briggs and Levine stated: "With the exception of affective neuropsychological symptoms, neurological symptomatology was virtually absent...Paresthesias were reported as a symptom in only five (3%) of the 145 cases, subjective limb weakness was reported in 2 (1%), and depression was reported in 18 (12%)."
- Dillon, M. J.; Marshall, W. C.; Dudgeon, J. A.; Steigman, A. J. (1974), "Epidemic neuromyasthenia: outbreak among nurses at a children's hospital.", British Medical Journal, 1 (5903): 301–305, PMID 4819151
- Dillon, M. J. (1978), "“Epidemic neuromyasthenia” at the Hospital for Sick Children, Great Ormond Street, London", Postgraduate Medical Journal, 54 (637): 725–730, PMID 746019
- Dillon, M. J.; Marshall, W. C.; Dudgeon, J. A.; Steigman, A. J. (1974), "Letter: Epidemic neuromyasthenia", British Medical Journal, 2 (5918): 559, PMID 4407290
- Briggs, Nathaniel C.; Levine, Paul H. (1994), "A Comparative Review of Systemic and Neurological Symptomatology in 12 Outbreaks Collectively Described as Chronic Fatigue Syndrome, Epidemic Neuromyasthenia, and Myalgic Encephalomyelitis", Clinical Infectious Diseases, 1994 (18 Suppl 1): S32-42, PMID 8148451, doi:10.1093/clinids/18.Supplement_1.S32