"until the humans are all dead"
Ok, so how does the human team actually win in this game?
Well, as an example, in Halo the winner of an Infection game is determined by who has the most kills. There are multiple rounds, and each round a different group of people start as zombies. In Halo it isn't really set up as a team-based game.
For this game, I could see it being set up where instead the human team wins if they reach a certain score, or if anyone survives to the end of the round. Something like that.