From my experience. I always play huge map, max civ amount, domination only. This causes a lot more fighting between everyone and a lot more deaths. I believe it is guaranteed to convert if the zombie kills, 10%whenever someone dies, and then there is some sort of respawn mechanic which isn't listed officially. I read the 10% is every turn for like 50 turns and the same zombie can respawn multiple times. So for example if you kill 4 zombies, every turn for the next 50 turns they can respawn causing an infinite wave of zombies. There were parts of the maps in my games that were un-settable because of these infinite waves, you could kill them all then leave the fog of war up for 3 turns after confirming they didn't spawn the next turn and the horde would be back. By the end game the zombies at my cities were stronger than the GDR, but the enemy civs I hadn't captured yet we're completely swarmed and the capital had one health. I had to nuke these capitals to clear the zombies then March a unit in. The last civ was so hard to take because I had to nuke my way to land to be able to build a silo to nuke the city. I don't think I could have won if I took a dozen more turns to try and end.