Employee’s coughing is bothering co-workers: What can we do?

Q. We have a very good employee who has a chronic respiratory illness and does not attend work (has doctor's excuse) when she has something contagious. She coughs incessantly about five times per year, lasting on to two weeks. We tolerate these episodes from a managerial/supervisory standpoint because it does not affect her work and she isn't contagious. Employees have complained that it affects their ability to concentrate as well as interfering with telephone conversations with clients. We need any suggestions that you may have as far as our legal stance on what can be done with the employee. -- Michelle, Georgia