A complex illness also known as chronic fatigue syndrome (CFS), chronic Epstein-Barr virus (CEBV) and myalgic encephalomyelitis (M.E.) that is characterized by incapacitating fatigue (experienced as exhaustion and extremely poor stamina), neurological problems, depression, anxiety, mood swings, difficulty concentrating, temporary memory loss, muscle aches and spasms, headaches, irritability, sore throat, sleep disturbances, fever and sensitivity to heat and light. The symptoms tend to wax and wane but are often severely debilitating and may last for many months or years.