One Foot in the Grave by Wm. Mark Simmons

"Unknowingly infected with half the combinant viral strain that transforms the living into the undead, Christopher Csejthe is in a state of suspended transition, neither alive nor truly undead, and has caught the attention of vampires, werewolves, and witches, who want to destroy him." (from Amazon.com)

Paperback Info: ISBN 0671877216, Baen Books, 1996