I am looking for the title of a science fiction book that was written in the 60' or 70's about a human space traveler visiting a planet. He goes into a cave and gets bitten by a creature that transfers its personality into him. He calls it Pard - short for partner. It appears that the man is now immortal and the book tells his story - even how he saved people from some sort of malady caused by another immortal. Anyone know the name/author? This has been driving me "nuts" for years now. Thanks.