An even uglier idea (i know it's not a competition, buuuut....)
The Astral Pigment used is tainted with a strain of HMHVV-X, giving rise to seemingly random HMHVV cases with no typical vectors in evidence. (Or, if you want to go "olde scole", insect spirit pheremones. Heck, why not both... just stat up some Mosquito spirits. Be the most despised GM in the 6th World.)
Even once the culprit(s) are discovered and stopped, there's a LOT of dark cleanup work to be done, and some of that may conflict with whatever sense of conscience the runners may have, despite being necessary. (Kill a ghoul today to avoid killing 10 next week? What about an edge case that has been exposed, but isn't showing symptoms(... yet)? What about an edge case that the character has known for a while, and likes?)