A Zombie is an NPC encountered in the Heist minigame. Zombies appear in the Burthorpe scenario, although only in the (Night) variation. Players on the Robbers team can take this form to disguise themselves and evade players on the Guards team.
Zombies are infected creatures. A player in this disguise can turn other NPCs into zombies as well. This means that, eventually, all citizens could potentially look the same, allowing the robber to blend in better.
When a zombie spawns, a message "An infected creature has spawned!" appears to all players in the Heist game.