Many people suffer greatly from disorders for which no specific cause has been identified. Some doctors believe these disorders are due to psychologic factors. Others believe that the disorders are caused by physical agents, such as viruses and toxic chemicals, or by abnormalities of the immune system. Although a cause has not been proved for any of these disorders, many people undergo considerable testing and try unproved treatments in time-consuming and costly attempts to diagnose and relieve their symptoms.

Last full review/revision February 2003