I think Cultists only get Chaos Undivided marks/icons. Typhus just turns them into zombies. While I like the idea, and would actually run them even though I don't like Nurgle, they don't play to my style well. Though if you want to scare the crud out of your opponent, at 4 points each Zombie, you could realistically take 3 full squads (90!) for 360 points. Lulz worthy.
Well at least now their weapons aren't worthless. But I'd imagine the Plague Marines' toughness is 5 now. What's the new strength for the DE weapons? 3? That means 6+ wounding. Ha!