Hypochondria is a psychological condition where a person experiences abnormal and chronic anxiety over imagined illnesses.
Reginald Barclay suffered from hypochondria, as did William Telfer. It could be treated with medicine or counseling. Telfer, however, was afraid of both, which left The Doctor with simply scanning him and offering him reassurance. Following Telfer's experience with a possible dark matter lifeform, however, he seemed to be cured. (TNG: "Realm of Fear", "Genesis"; VOY: "Good Shepherd")