Functional disease refers to a medical condition in which the body or a specific part of it doesn't function as intended without any visible physical, structural, or biochemical abnormalities. Synonyms for the term include functional disorder, non-organic disease, psychosomatic illness, and somatoform disorder. Functional disorders can affect any part of the body, including the digestive system, nervous system, respiratory system, and musculoskeletal system, among others, and may cause a range of symptoms such as pain, fatigue, weakness, and cognitive impairment. Treatment for functional disease typically includes symptom management and addressing any underlying psychological factors that may contribute to the condition.