**Symptom description**

*Prolonged fatigue:* persistent profound fatigue or lack of energy, usually (but not necessarily) accompanied by other symptoms; should be present for at least one month *Chronic fatigue (CF):* persistent fatigue or lack of energy, that leads to reduced activity levels lasting over 3–6 months\*. This may be explained by a condition other than ME/CFS (e.g., cancer-related fatigue) or unexplained ("idiopathic chronic fatigue"). It does not require other symptomsthataretypicallyfoundinME/CFS

*Post-infectious fatigue or post-viral illness (PIF or PVI):* new onset symptom complex including persistent profound fatigue with exercise intolerance following an infectious trigger and which is not otherwise explained by a diagnosed condition or lifestyle. It is usually accompanied by at least 2 further symptoms\*\* from: post-exertional malaise, unrefreshing or poor sleep quality, cognitive or autonomic symptoms for at least 3 months (i.e., this is a subset, where the viral aetiology is clear, of patients with chronic fatigue).
