There are essentially three different theories that are used to prove discrimination against people with disabilities: disparate treatment- that a person has been treated differently because of membership in a protected class - may be proved by direct evidence of discrimination or by inference. Today, employers are often open about discriminating against people with disabilities. They frequently know little about disabilities and make their decisions based on stereotypes rather than on individualized assessments. Further, medical examinations and inquiries are required by the ADA to be conducted after a job has been offered thereby enabling job applicants to determine that their disability was the determining factor in the hiring decision. Once an employer admits that the individual was treated differently because of a disability, a prima facie case of discrimination has been established and the question then becomes whether the discrimination was unlawful.
Ellen M. Saideman, The ADA as a Tool for Advocacy: A Strategy for Fighting Employment Discrimination against People with Disabilities, 8 J.L. & Health 47 (1993-1994)