According to the EEOC, the ADA covers employers with 15 or more employees, including state and local governments, and a covered employer must make a reasonable accommodation for the known disability of a qualified applicant or employee unless it would impose an undue hardship.

In the source's own words

“The ADA covers employers with 15 or more employees, including state and local governments. ... An employer is required to make a reasonable accommodation to the known disability of a qualified applicant or employee if it would not impose an "undue hardship" on the operation of the employer's business.”

Fact Sheet: Disability Discrimination, U.S. Equal Employment Opportunity Commission, 1997, Fact Sheet: Disability Discrimination, coverage and reasonable accommodation paragraphs.