THE YEERKS
The Yeerks are a parasite species, they enter through your ear canal and then connect themselves to your brain. From then on you are a controller which means you have no control over what you do, say, touch or eat. If you want to eat and are really hungry, you can't until the Yeerk decides to. Same with going to the toilet, climbing out of bed and you can't look at want you want to. The Yeerks can sift through your memory so they can pass as you, and sometimes they watch memories for amusement. All this time they are trying to get other humans to controllers. You are completely controlled until the Yeerk exits your brain, which it has to do every three days so it can soak up Kandrona rays in a Yeerk pool. When the controllers are free they are placed in cells and have no real chance of escape although every know and then there is an attempt but rarely do they work. The Yeerks home planet still has Yeerks on it but most are in host bodies trying to take over other planets.
The Yeerks were once confined to their own planet but along came Prince Seerow, an Andalite, and because he felt sorry for the Yeerks as they were very intelligent and curious of the world, he gave them some Andalite technology and the Yeerks developed weapons and spacecraft from them. Soon the Yeerks had taken over the Gedds, the Taxxons, who had all chosen to be controllers but a Yeerk could never control a Taxxon properly because of it's insane hunger and following them were the Hork-Bajir, the Andalites tried to help here but instead it only wiped out Hork-Bajir not the Yeerks and many managed to escape. The next target was Earth because of it's population. Visser 3 was put in charge and the rest is, well obvious.
In their natural state the Yeerks look like overgrown slugs. They are completely helpless as well as deaf and blind. some Yeerks only take a host to see the world and then never leaver their host apart from feeding. Like I mentioned before every 3 days they go to a Yeerk pool to feed on the Kandrona rays. The Yeerks think they are very superior and are convinced they will take over the whole universe. Other races think not.
The Yeerks have a special front, The Sharing which contains Yeerks. Normal humans go to meetings and are told about how wrecked and sad kids lives were until they joined The Sharing and how it's really one big, happy family. At the meetings they play games, pool, eat food and drink drink and then there is a meeting for 'full' members. Translation: Controllers. After two meetings you have to make your decision of whether you want to be a full member or leave The Sharing. Most people love The Sharing by this time and join, to get a fat, ugly, overgrown slug stuffed into their head.