Myalgic encephalomyelitis (ME) is characterised by a range of neurological symptoms and signs, muscle pain with intense physical or mental exhaustion, relapses, and specific cognitive disabilities. Early reports dating from 1934 described epidemics of the illness (see Research Publications on ME epidemics for more information) — such as the 1955 outbreak at the Royal Free Hospital in London — but nowadays it is more common for endemic (sporadic) cases to be identified. ME is classified by the World Health Organisation as a neurological illness (International Classification of Disease 10: G93.3).

The cardinal symptoms of ME are profound, generalised post-exertional loss of muscle power (fatigability); muscle pain that may include tenderness and swelling; and neurological signs. Patients are also prone to relapses which may take the form of recurrences of the original systemic illness, or fresh episodes of muscle weakness, neurologic changes or well-defined cognitive problems. As with many chronic illnesses, fatigue may be present, but in ME patients the fatigue is post-exertional, often delayed, and quite unlike the ‘fatigue’ experienced by healthy people.

The cause of ME is still unknown, but there is unlikely to be one single causative agent. Several early epidemics appear to have been triggered off by an outbreak of an infection with enteric organisms or poliovirus which then subsided, and many patients report an infectious onset to their illness. However, in others, there may have been a variety of contributing factors — infectious, traumatic, environmental and endogenous — all of which can lead on to a condition which shares a set of common symptoms.