Degenerative xenosis is a medical condition that arose when races became star-faring, quite a serious disease. This medical condition causes a degeneration of the neural pathways, and as the nervous system cannot naturally regenerate, there was no known cure for it. It was believed that races that lived in "alien" environments would suffer from such a disease, which was the case with Spock's mother, Amanda Grayson.
However, the Vulcan Academy of Science had been developing an experimental procedure that would combat this disease. It was developed by the Vulcan Healer Sorel and his associate, Doctor Daniel Corrigan. Corrigan, suffering from the disease as well, tested the nerve regeneration treatment on himself and recovered. (TOS novel: The Vulcan Academy Murders)