<qa>  What is a chronic musculoskeletal pain disorder of unknown etiologyFibromyalgia, a chronic musculoskeletal pain disorder of unknown etiology, is characterized by widespread musculoskeletal pain, fatigue, and multiple tender points; the disease affects 3 to 6 million Americans. A person is considered to have fibromyalgia if he or she has widespread pain in combination with tenderness in at least 11 of 18 specific tender point sites. 