I was playing vamps and my brother played {contagion engine}. When he tried to proliferate my guys off I respond with {urge to feed} and tapped my guys to give them +1 counters. Did my vamps still have counters when contagions ability resolved?
No
Yep the -1/-1 counters and +1/+1 counters cancel each other out when the state is checked and they remove each other.
BUT if you have a creature with undying and a +1/+1 counter and it receives enough -1/-1 counters to kill it, it dies before the counters cancel each other.
Example:
1- Your {strangleroot Geist} has a +1/+1 counter, then your opponent casts {black sun's zenith} for 1. The next time states are checked, your Geist has a +1/+1 counter and a -1/-1 counter on him. They remove each other. Now he can die and come back again with undying
2- Your {strangleroot Geist} has a +1/+1 counter, then your opponent casts {black sun's zenith} for 3. The next time states are checked, your Geist has a +1/+1 counter and three -1/-1 counters on him and his toughness is 0. He dies and doesn't come back, because when he died he had a +1/+1 counter.
Hope I could explain it
Mentonin, I couldnt have explained it any better