For the safety of the passengers and crew of an aeroplane, it is imperative that an airline pilot maintain a high level of fitness. There are a number of medical conditions which, once diagnosed, may prevent a pilot being allowed to fly a plane. If they can be stabilized, some conditions may allow a return to work. Other conditions are classed as 'non-medically certifiable'. Upon diagnosis of a non-medically certifiable condition, a pilot's medical certificate will be denied, and if already issued, it will be revoked.