Hmmm....aside from grave diggers and funeral homes who are pissed off about not getting paid??!!
Maybe this is the GOD of DEATH's (insert specifics here) way of punishing those who have escaped his domain. If i keep getting screwed out of powerful additions to my forces/power control, i might either invent the virus or have invented for me to punish the trangressors. Although in this situation, i might have the virus either kill, or better yet, cause massive depression so the carrier wants to kill himself and return to the afterlife!
Another possibility could be the Necromancer theory, with a twist. Its is a spell effect that is created by an Arc Necro to pervert anyone who had been ressurected to transform into obediant servants. All he has to do is identify those characters would had been ressurected.
If its a natural virus, there would had have to of been some natural event that happened that triggered the virus. Otherwise the virus would have happened the first time anyone was reesurected.
I think you have to create some specifics before it can really take off. Does this always happen to ressurectees (sp?!). If not then its either targeted or is something that is happening now because of an event that has just triggered it.
It would probably also make a difference how the the character was ressurected. Ressurection, True Ressurection, or Reincarnation. It could also have been from an artifact or direct divine influence (DDI).
DDI - I doubt it could happen as a result of a DDI because its using the power of a god. Unless of the GOD himslef wanted it to happen, or he got trumped by a stronger god.
Artifact - It could easily be byproduct of the artifact. Especially if the virus transform the character into a template. thats classic. It all depends on the origin of the artifact.
Ressurection - This could be a natural phenomenon because you need a body part to perform the spell. Perhaps its possible that the the virus is the result of a body part the constitutes the new body, but had just enough decay to create a virus. That seems kinda cool.
True Ressurection - This is not likely the result of natural virus and would require an outside source. Spell interfernce or the old divine gag!!
Reincarnation - This is pretty much as above, although since its source is druidic/nature based, i can't see the virus then being a natural occurence. I don't think nature would disagree with itself!
All right, that turned out WAY more long-winded than i had planned!!!