Having been diagnosed with Parkinson's 15 years ago at age 36, I today still have no mental limitations and I can concentrate the same way as I could before the diagnosis (as has been tested several times by neurologists). Among other tasks, I am still programming and leading development teams. It seems to me that programming is the ideal job for someone with Parkinson's since one can do it independent of the fact that some motoric skills such as walking are reduced or one's voice is not so crystal clear anymore. I learned from my own experience that an illness or handicap does not mean to drop out of the professional world, in contrary I made very good experiences with ill/handicapped colleagues and employees since they are usually very dedicated. The most important thing from my view is to address any and all issues related to such a condition in an open, respectful manner so that your ill colleague does not feel pressed to hide problems and ask for help concerning things like commuting to work etc. (sorry for my inelegant English, it is not my mother tongue).