In reality, many of the doctor's diagnoses were false, with his treatment instead being designed to alter the memories of his patients. These patients were actually the crews of starships, including the USS Voyager, abducted as part of a government conspiracy in 2377 to reduce a severe manpower shortage on Quarra.
They were supposed to remember nothing of their former lives. If a worker's memories began to return, as happened to Tuvok, Kadan would repeat the process, thus brainwashing them again. (VOY: "Workforce", "Workforce, Part II")