From "Animorphs: Underground (#1.3)"
Jake, I'm really sorry about your brother.
I'm gonna set him free no matter what it takes. When we do, I'm gonna take that Yeerk inside of his head and squash it like a bug.
At least now we know that the Yeerks need some sort of booster treatment every three days.
And we know a way into this pool, whatever that is.
Yeah, you'd think that Elfangor would've given us an instruction manual or something.
You mean before or after he went toe-to-toe with Visser Three?