Brain slugs

Brain Slugs are little parasitic slugs that attach themselves to the heads of intelligent beings (such as humans) and control them. They are about as big as a fist, green, translucent and have two feelers and one enormous eye.

Symptoms of Posession
A person possessed by a brain slug will be likely to speak in a monotone voice and do everything to further the spreading of brain slugs throughout the universe, but the easiest way to tell a person is infected is by the brain slug sitting on their head. The person possessed retains awareness of his condition, which Hermes referred to as a "nightmare". The recommended procedure of dealing with a brain slug victim is to act natural and switch to a garlic shampoo. Another way to deny attachment is wearing a helmet. Fortunately, brain slugs are not very intelligent and can be easily removed without harming the possessed person, minimizing the risk. Once removed from the afflicted persons' head, the person returns immediately to normal.

Brain Slug Party
There is a Brain Slug Party on Earth, naturally organized by citizens of Earth with brain slugs attached to them. Its political goals include awarding unreasonably high subsidies to the Brain Slug Planet and attaching brain slugs to "the working man"- and basically everyone else on Earth.

Characters Assimilated
Two employees of Planet Express have had experience with brain slugs. Hermes was afflicted in "A Head in the Polls", and again in "Raging Bender" when he managed to infect Fry. In Fry's case, the brain slug died of starvation shortly after attaching to his head. This raises interesting questions about whether the brain slugs feed on the victim's Delta Brain Wave to survive, for Fry lacks this particular wave form. (3ACV07) However, it is also possible that Fry is just an idiot and lacks the mental energy to sustain an adult brain slug. In the first comic of the series, the Slug-O-Vac removes a brain slug from Fry's brain, which would debunk previous theories as Fry had been affected. The non-canon status of the comics makes Fry's ability to be assimilated uncertain. In the game, Hermes is again infected.

Trivia

 * On the audio commentary for "Raging Bender", David X. Cohen states that the brain slugs were initially planned to fulfil a different, larger role. They would have hung on ceilings and dropped on the heads of people, making them smarter for a time, but ultimately covering their entire heads.
 * A brain-slug-like hat is featured in Valve's Team Fortress 2 and is named Triboniophorus Tyrannus after a green Australian native bug.
 * Their home planet is located in the "Spleef" nebula or possibly somewhere between there and Earth.