The game also features an infection system, which determines whether or not an NPC is infected by the Thing. Although most NPCs are scripted to transform into a Thing at specific points in the game, they can also be infected at any time prior to this. When the team comes under attack by enemies, the possibility of infection is based upon a probability system whereby any teammate who comes into direct contact with an enemy can be infected. Although infection does not alter their immediate behavior, it will result in them turning into a walker-Thing after a set period of time, at which point they will attack the team.