Chronic disability following a presumed viral infection. The most characteristic and persistent feature of the disease is muscle fatigue unrelieved by rest. Other symptoms include low-grade fever, headache, blurred vision or diplopia, stiff neck, vertigo, nausea and vomiting, lymphadenopathy, emotional lability, insomnia, urinary frequency, and either deafness or hyperacusis. In addition depression, anxiety, or other psychological symptoms may be present.