Yeerks are a fictional extraterrestrial species from the book and television series "Animorphs" written by K. A. Applegate and published by Scholastic.
A Yeerk is a parasite that takes over the body of another being by way of infestation through the ear canal. It resembles a small, grey-green slug about two inches long. Yeerks in their natural state have decent senses of smell and touch, but they are blind until they enter a host, as they do not have eyes. They possess antennae-like protrusions called palp, as well as osmosis node, small organs that process nutrient in the Yeerks' native pools. Although the presence of a circulatory system of some kind is debatable, apparently Yeerks have no heart, as evidenced by the description of a human morphing into a Yeerk, in which the human heart stops and dissolves away completely.
Yeerks communicate in their natural state using a language of ultrasonic squeaks, and use sonar to get a basic picture of the surroundings. The process of infestation by a Yeerk is not totally painless, but it does not hurt as much as it seems like it should, as a numbing chemical is secreted to anesthetize and dilate the ear canal itself so that the slug-like Yeerk will not cause any physical harm to the host. It is able to manipulate and puncture a variety of alien ear canal structures including bone and membrane to reach the brain. As the yeerk reaches the eardrum and other ear canal structures, it pushes them aside.
After the process of infestation, the Yeerk flattens itself into what has been described as "hammered-out gum" and sinks into all of the cracks and crevices of the brain. From there, all control is submitted involuntarily to the Yeerk (however, the host may fight and very rarely win). At this point, the host has no control over itself, no secrets, and no privacy. The Yeerk can even play with the memories of its former host. Although control can be returned to the host by the Yeerk, this does not occur often as most Yeerks prefer their parasitic relationship to a symbiotic one.
Read more about Yeerks at Wikipedia ...
Official Site: K.A. Applegate
Links: Wikipedia Animorphs Reference Page Seerowpedia