I've wondered this ever since I started watching Mushishi, and the issue is brought up again in ep 20. Is Ginko immortal? Immortality is definitely possible as shown in one of the earlier episodes where a guy eats the tooth growing on the roof of his mouth and becomes immortal. At the end of ep 20, Tanyu keeps saying how Ginko will still be alive even by the time she becomes an old woman. It seems like everything indicates that Ginko had some incident with a mushi and became immortal.