Creep is the thick viscus substance left behind by the infestation. It is a nutrient filled sludge that can sustain the growth of zombie eggs and infected plant matter. When large enemies are killed, such as Infestors or Agrons their internal organs will spill out, causing a pool of creep to form on their corpse.
Creep has two basic functions. The first it is greatly slows player characters, with the exception of the Technician, who is robotic and unaffected by creep and the Pathfinder who is unaffected due to his jumpjets. The Rifleman has adaptive leg augments that reduces the effect the creep has on the movespeed but not completely preventing it. The second is it speeds up the zombies, and in combination with charging on Nightmare, can cause them to move at almost inescapable speeds.Note that creep gives shared vision with zombies so be careful on survival.
Almost all instances of creep can be killed. Within all patches of creep there is a source, this source is often vulnerable, even if not target able. Weapons like the Combat Knife, Stinger MK-3, and skills like Grenades and Tactical Nuke can hit creep through their splash. Players should actively attempt to purge all creep they come across, to put them on a more even playing field with their foes.