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.