The Americans with Disabilities Act (ADA) of 1990 makes it illegal for employers with 15 or more employees to refuse to hire people with disabilities if making "reasonable accommodations" would enable the person to carry out the duties of the job. That could mean making physical changes to the workplace or reallocating certain responsibilities.
While the law is unclear on exactly how far an employer must go to accommodate a person with disabilities, what is clear is that it's the applicant's responsibility to tell the employer about the disability. Employers are not allowed to ask whether an applicant has a disability or a history of health problems. However, after the applicant has been given a written or verbal explanation of job duties, you may then ask whether he or she can adequately perform those duties or would need some type of accommodation.