Wandering Zombies are scattered around the Survival Ops map and ou can fight them to earn more money, intel or glory. You can also choose how difficult missions you want to play - easier missions have only 1 zombie in the icon and offer lower rewards. The medium difficulty has 2 zombies in the icon and requires more powerful weapons. Finally, the Bloody difficulty zombies have 3 zombies in their icon and require a special skill - the Ney Your cleaner - to complete.