In summary, it can be said that the admissibility of such a questionnaire depends on the position or function. In case an applicant has to fill in a questionnaire, he/she has to declare his/her illnesses (e.g. diabetes), but the doctor is not allowed to inform the employer about a diagnosis. He may only inform about an unsatisfactory suitability for the job, for example, if the illness directly and currently affects the ability to work or makes it impossible to perform the job.
Source: FDPIC – Health questionnaire in application procedures