“❮Parasitism is a limiting choice. The Yoort moved violently to conquer other species and infest them, but this was not profitable, not in the long haul. So the Yoort used biological engineering techniques to design and create a species specifically to be a symbiote.❯ ❮Who cares how you did it?❯ Tobias argued. ❮So you build the Isk and then enslave them.❯ ❮No, no,❯ Guide pleaded, whining away through his diaphragm. ❮The Isk were true symbiotes. The Isk cannot live without the Yoort. And to ensure that this symbiosis would be real, the Yoort, too, were modified. Now Yoort cannot live without Isk and Isk cannot live without Yoort. They are one creature with two parts.❯ Dead silence. No one said a thing. The reality of it was sinking slowly into our suspicious brains. ‘Oh, my God,’ Cassie said at last. 'Of course. It’s the way. The only way. Parasite becomes symbiote. No more infestation. They create the next step in their own evolution and become true symbiotes.’ 'No more war,’ Erek said quietly. 'No more need to conquer new species, to infest and enslave.’ 'The Yeerks don’t know about this,’ Cassie said. 'Even the Yeerks who want peace cannot imagine a way out, a way to end the cycle of conquest.’ ❮These Yoort could be related to the Yeerks,❯ Ax said. ❮They may be the same species, somehow separated long ago, perhaps carried from the Yeerk home world by some forgotten race.❯ ❮If the Yeerks knew…if the Iskoort ever made contact with the Yeerks…❯ Tobias said. This was why Crayak had to destroy the Iskoort. And why the Ellimist couldn’t allow it. Someday, maybe far in the future, Iskoort would meet Yeerks. And the Yeerks would see that there was another way.”
— Book #26: The Attack (Jake), pg. 94 (by K.A. Applegate)