There is no set list of conditions that constitute a disability. The test is simply whether an employee's condition "impairs a major life function."
According to the most recent Supreme Court decision, this analysis requires the courts to review whether the person is able to perform the tasks of daily living (washing, brushing teeth, fixing meals, housecleaning, etc.), and decide if the person is significantly more impaired in those tasks than other persons in the population who are not “disabled.” For more information on what constitutes a qualifying disability under the ADA (for whicht he same analysis applies), visit this link: http://www.ada.gov/qandaeng.htm
As noted, an employer with less than 5 employees is not bound by any state of federal law that would restrict their ability to terminate a sick or injured employee.
Further, unless some sort of work-from-home arrangement can be entered into, it is unlikely that any accommodation of this employee's disability would be deemed "reasonable," as you presumably need this employee in your office and not having them or someone to take their place will cause harm to your business.
If you require any further clarification, please do not hesitate to ask. Otherwise, I would be very grateful for a positive rating of my service so that I may receive credit for assisting you.