The Yeerks are a parasitic species of slug-like creatures who can only exist in nutrient-rich pools or marshes which are exposed to Kandrona Rays. They control a galactic empire, and are attempting to enslave the entire galaxy. They are opposed by the Andalites.
They can crawl into the brains of humanoid creatures through the ear canal. From there, they control the host creature's body, until they have to leave the brain to recharge on Kandrona Rays- once every three days. If they don't leave, they'll die.