ANSWER: No. You have a right to disability retirement even if you were sick or injured prior to your government employment, so long as you became disabled by the symptoms of your disease or injury after you were employed. In other words, a pre-existing disease or injury will not bar you from collecting, as it might with many types of disability and health insurance. A pre-existing disability, however, will bar you from collecting.
Example: Charlie entered government service with cerebral palsy, but he was able to perform his job very well for a number of years. Later, his condition worsened so that he lost the ability to perform some of the critical elements of his job. Because he is unable to perform at least one of the critical elements of his job, he is unable to render "useful and efficient service" and is legally disabled. Although he entered government service with cerebral palsy, the condition did not become disabling until later. Thus, if he meets the other requirements, he is entitled to disability retirement.