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1.
An alien parasite, the villians of the Animorph series, written by K.A. Applegate. They closely resemble large earth slugs and are grayish-green in color. They enter the brain through the ear canal and take over the host body completely. Beings taken over in this manner are known as Controllers. The Andalites have been their sworn enemies for more than 39 earth years.
Andalite: "Yeerk filth!"
by Malani October 06, 2006
 
2.
A slug-like parasitic alien. The Yeerks go from planet to planet taking over civilizations for host bodies. Without host bodies, the Yeerks are nothing but space slugs.

The Yeerks crawl into your ear and wrap themselves around your brain, thus being able to control your body and gain access to your memories. They have come to Earth.
They could be anyone. Your friends. Your family. And if they have not taken you yet, they most likely will.
The leader of the operation of taking over Earth is Visser One, previously Visser Three.
The only race opposing them is the Andalites. However, there is something being done about Earth. There are five kids and one Andalite, named Jake, Rachel, Marco, Cassie, Tobias and Aximili (Ax for short), who battle the Yeerks. How?

With the power to morph, that is, change form from one creature to another like Beast Boy from Teen Titans. They recieved this power from a dieing Andalite prince named Elfangor.

The Yeerks first thought the Animorphs were no threat to their mission, but they soon find out different. I hate Yeerks.
The Yeerks and Animorphs are in a book series called.. well, The Animorphs, written by KA Applegate.
by GalaxyDancer May 22, 2005