In 1968, after landing on the Hork-Bajir homeworld, Akdor 1154 had three Hork-Bajir captured, with one being infested by Esplin 9466. After Esplin confirmed that the numerous Hork-Bajir hosts could be used as shock troops and weapons to fight the Andalites, Akdor created the Yeerk Empire and began mass infestation of the species.
Council of ThirteenEdit
The Council of Thirteen is at the top of the Yeerk hierarchy. One of the members of the Council is the Emperor, but only the Council members know which one. There are a variety of hosts on the Council including Hork-Bajir and Taxxons. There are no humans on the Council, and no Andalites as Visser Three/One was never appointed to the Council.
Just below the Council of Thirteen are the Vissers, generals of the Yeerk empire. The lower the number the higher ranking the Yeerk, therefore Visser One is the highest, and this goes on down until Visser forty-seven or so.
Below Vissers are Sub-Vissers. They are the colonels of the Yeerk empire, having subordinates but still taking orders from a superior officer. The same applies to Sub-Vissers as Vissers, the lower the number the higher the ranking. There are about 200 Sub-Vissers according to The Andalite Chronicles. In "VISSER" it is revealed that there are over 400.