Ok, maybe this is just wishfull thinking but Festus's pestilent potions rule give "any unit he joins" 5+ regen and poison attacks.

Now it doesnt say they loose that ablility if he leaves the unit or dies and its not a spell, there potions the unit take (but that may be a little fluffy)

Either way, it says "any unit joined by Festus", not "any unit with festus in" or "while festus is in the unit"

So surley he could mosey his way through all units in the army giving everyone poison and regen?

Tad good i know but i cant find anywhere, anything to contradict this?