It doesn't matter if Zombies have the mark. Typhus does, and he is the one causing models killed, which is what triggers the tally.
Yes, I know. Its "neutral" and the rules say that you can shoot and assault it on its own. I mean, you can be TFG and do the trick, but personally I think it's a waste of points and a scoring unit. Personally, I think you can't because you purchased it out of your Force Org.
Plus, there's no guarantee you will get Epidemus on the board first turn so it's potentially pointless.