Personally, I'd trade Virulence for Epidemic in the unholy tree. That would give you the extra time to burn through another rotation without worrying about your viruses wearing off. thats just me though.
Although, I wonder if Improved Frost Presence is worth it.....I've put 2 points in it, but now I'm starting to wonder. If you decide against it, I reccomend Two-Handed Weapon Specialization. The 4% damage is worth the TPS, imo