Yeerks are fictional parasitic aliens from the Animorphs book series created by K.A. Applegate. They are slug-like creatures that can enter the brains of other beings, taking control of their bodies and minds. Yeerks are known for their ability to manipulate their hosts, using them to achieve their own goals. In the series, the Yeerks are depicted as a significant threat to Earth and its inhabitants. They are part of a larger conflict involving the Animorphs, a group of young humans who can transform into animals. The struggle against the Yeerks highlights themes of control, freedom, and resistance.