When employees suffer a disability, the ADA requires you to offer a "reasonable accommodation" to help them perform the essential functions of the job. Some disabled employees may request that they can work from home.
Telecommuting may be a workable solution for some positions. However, you can say "no" if the job requires direct supervision or the employee's physical presence at work.
Remember, as the employer, you get to pick the accommodation. It doesn't have to be the accommodation the employee prefers; it just has to be "reasonable."
Recent case: Cynthia took an eightmonth leave from her college library job to deal with depression. Upon her return...