The Yeerks are a group of parasites that takes over a living body and controls it. Yeerks can also read your thoughts, feelings, and your memory. Seerow, an Andalite gave the Yeerks some advanced technology and the Yeerks started to try to control other species. The Yeerks have a special class. The Vissers. The Vissers are highly promoted Yeerks that have done something well and that the Council of 13 approves it. The Council of 13 is like the high leader and judge of all Yeerks. The Yeerks travel to other planets and take over the species that live in it. So far, they have took the Hork-Bajirs, Taxxons, Gedds, Some humans, and one Andalite. Yeerks are just yucky gray slugs that crawl to your brain filling all the gaps in it, and taking over your whole body. You can try to resist the Yeerk in your head, but it won't work. The Yeerk will always be able to conquer you if you already have a Yeerk in your head. Yeerks need special rays to eat. They absorb rays from their home planet's sun, but since Yeerks travel to other planets that don't have rays, they have a Yeerk pool. The Yeerk pool gives special rays called Kandrona Rays to the Yeerks. Without the Kandrona Rays, the Yeerks will die. They have one in every planet and they also have a spare Yeerk pool at their mother ship for the high classed Yeerks. A Yeerk has to absorb Kandrona Rays every three days or they will die. While Yeerks are in the Yeerk Pool, they are blind and absolutly helpless unless a living creature accidently falls or soak it's head in a Yeerk pool long enough for a Yeerk to get into it's brain.