Inclusion Criteria:  Diagnosis set within Stockholm County of ME/CFS, burnout syndrome or post-COVID-19-syndrome according to the Swedish version of the international classification of disease (ICD)-10 Control group (rheumatic disease): Diagnosis of rheumatoid arthritis Control group (healthy): no diagnosis related to inflammatory disease or fatigue  Exclusion Criteria:  Organic or neuropsychiatric disease that explain the fatigue among cases diagnosed with ME/CFS, burnout syndrome or post-COVID-19-syndrome according to the Swedish ICD-10