Parasites are enemies in Call of Duty: Black Ops III that appear on the Zombies maps Shadows of Evil, Dead Ops Arcade 2: Cyber's Avengening, Revelations, and throughout the Nightmares campaign.
In Shadows of Evil, Parasites are yellow-colored flying creatures that attack the player in various numbers. Much like Hellhounds, Parasites mostly only appear during their own special round and the last one killed drops a Max Ammo. However, they also appear after a Margwa's head has been destroyed and randomly from a harvest pod. They attack by shooting a yellow liquid at the player which covers the screen, impairing vision slightly, and dealing damage.
In Dead Ops Arcade 2: Cyber's Avengening, they act mostly like they do in Shadows of Evil, but only appear after one of the Margwa's heads are destroyed.
In Nightmares mode, they are purple instead of yellow. Their difficulty varies, but there are several instances where they are extremely tough to fight, even on lower difficulties, because they shoot their liquid out at a very fast rate and stay high up in the air. Therefore, seeking or staying near cover may be necessary in order to defeat a Parasite. The rest of the time, they are generally very easy to fight, as they sometimes don't shoot at the player with the liquid at all. They have very low health in all missions, only taking a few shots to kill even with low-damage weapons. It is also possible to melee Parasites when they are in close range.
In Revelations, they behave the same as their counterparts from Shadows of Evil, but they are red and they shoot red liquid instead of yellow.