A couple of the early novels strongly suggests that HMHVV is from the Fourth World. There is the wendigo who turned Twist's sister that basically hibernated through the fifth world, and in Nosferatu Serrin goes up against an elf Nosferatu that is several hundred years old, at least. While the elf could have been a spike baby, the virus clearly had to find him at some point, which means it was already here before the awakening, though perhaps lying dormant with the lack of magic.
Isolated instances of HMHVV outbreaks at mana spikes would also explain cultural references to vampires, werewolves (loup-garou), and the like.