The decision to walk away is entirely yours. But before you make any rash decisions, maybe take a moment to put things in perspective. Yes, herpes and Hep C are tough diagnoses. The truth is, your husband might not have known he got herpes or Hep C. Sometimes they can lie dormant in the body for years. There may be no symptoms for years.
Do you really want to punish him for a mistake he made 28 years ago? Do you love him? After this many years together, it seems like you would have developed the communication skills necessary to work through this difficult time.
It is only the end of the world if you decide it is. But don't punish him, and yourself needlessly. There is nothing you can do about it now.