Myalgic encephalopathy (ME) or Chronic Fatigue Syndrome (CFS) can overlap with other disorders such as a primary sleep disorder or depression.
According to the NHS, the main symptom of ME/CFS is extreme physical and mental tiredness that is not helped by rest or sleep.
Colin Barton, chair of the ME Society, said: "We are fortunate in Sussex to have a very good NHS specialist service in Haywards Heath for adults where GPs can refer those suspected of suffering from ME for diagnostic assistance.
"There is also a good service for youngsters with the illness based in Brighton.
"ME that can sometimes follow a viral infection or trauma affects over 4,000 people across the county many of whom are housebound and in need of care.
"Symptoms include profound physical and mental fatigue, concentration and working memory difficulties along with sleep and mood disturbances."
Other symptoms include muscle or joint pain, headaches, problems thinking, remembering or concentrating, flu-like symptoms, feeling dizzy or sick, heart palpitations, and a sore throat or glands.
Sussex ME Society say that early diagnosis and good management of the condition can often lead to significant improvements and avoid severity and chronicity of the illness.
They are working to emphasise the importance of people being given a thorough assessment, preferably at a specialist NHS centre, to exclude other possible illnesses before being formally diagnosed with ME.
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Have you been affected by ME or CFS? Please email [email protected]