Undeath Contagion

All zombie creatures have Vigilance and Infect.
Zombies cannot be blocked by other Zombies. If a creature with a -1/-1 counter on it dies create an X/X black Zombie creature token that's a copy of that creature where X is half that creature's total toughness, round down each time.
At each end phase put one -1/-1 counter on all creatures with -1/-1 counters on them.