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. (TNG: "Realm of Fear", "Genesis"; VOY: "Good Shepherd")
In Photons Be Free, the holonovel written by the USS Voyager's EMH, the character based on Harry Kim, Kymble, was a hypochondriac. (VOY: "Author, Author")