The origins of the Yeerks are never made clear. The earliest event in Yeerk history presented is simply called the Cataclysm, which was never fully understood by the Yeerks. The Cataclysm was a disaster, the nature of which was forgotten long ago. It resulted in half of the Yeerk population being removed from their homeworld and taken out into a distant region of space. In Neomorphs, it is revealed that the One was responsible for this, as he needed some Yeerks to control his Kelbrid army. For more information, see Kelbrid and Yoort.

The Yeerks existed in ignorance on their homeworld for a long time until the Andalites arrived. The Andalite leader, Prince Seerow, learned that the Yeerks were intelligent and began teaching them about space travel, science, technology, and other races. After learning much from the Andalties, the Yeerk Counsel of Thirteen made the decision to betray the Andalites. A small force, reported to have been led by Guraff 427 attacked the Andalites and stole some of their ships and weapons. After retrieving a supply of fellow Yeerks from the homeworld, those ships spread forth into the galaxy to begin the Yeerk Empire.

The Empire spread across the galaxy, and many races were enslaved fully or partially by the Yeerks, including the Hork-bajir, Taxxons, Gedds, Nahara (never seen), Leerans, Garatrons, Humans, Sstram (never seen), and the Mak (never seen). Only three races offered an active offensive atainst the Empire, the Andalites, Anati, and one other as yet-unnamed species.

Eventually, the Yeerk Empire collapsed due to a combination of internal pressures, failed governmental policy, and military losses. The infighting of he various Vissers stripped the Empire of some of it's greatest leaders and most brilliant minds. In addition, the Yeerks suffered extremely heavy losses during their failed invasion of Earth (thwarted by the Animorphs and the focus of the canon series) and even greater losses due to a similarly failed attack on the Anati homeworld. The Yeerks were left with no choice but to ask the Andalites and Anati for peace terms.

The result was a thorough blockade of the Yeerk homeworld and several other Yeerk strongholds, such as Hork-bajir and the Taxxon world. This blockade lasted for several years. All of the Yeerks were either captured, imprisoned, or killed. However, a small group of renegades escaped beyond Andalite reach and into Kelbrid Space. there, they encountered the One, who assumed command and sent them back to Earth to invade again. He equipped a good number of them with Kelbrid hosts.

During the invasion, the Yeerks released Esplin 9466 from prison, and he assumed command of the Yeerks. In book 68: The Redemption, the Yeerks established an alliance with the Pythagi Conglomerate. In A New Foe, the Yeerks, with help from their Pythagi allies and Kelbrid soldiers broke the blockade around their homeworld and established the New Yeerk Order. From there, the Yeerks began another attempt at galactic conquest.


Yeerks are small, parasitic slug-like life forms capable of infesting other races and taking control of their motor functions. Wthout a host, Yeerks have almost no senses. They are blind and mostly deaf. Yeerks also have the ability to read their host's memories, allowing them to pass flawlessly as the host. The goal of the Yeerk Empire and the New Yeerk Order is to gain as many host species as possible so that their entire race will eventually possess host bodies. The other goal, of course, was to dominate the entire galaxy.


Though there are a few notable exceptions, most Yeerks are power hungry and willing to betray their superiors and even put the Empire/Order at risk for their own gain. Yeerks rarely understand such concepts as loyalty, friendship, love, and others in that general vein.

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