The Sickness (Animorphs #29)

Something is very wrong with Ax. He's sick, and the Animorphs don't know what to do about it. They can't take him to the hospital. They don't know how to contact the Andalite home world. And Ax is dying.

Cassie and the other Animorphs come up with a plan. But it involves finding a new way into the Yeerk pool. There's just one problem: Jake, Rachel, Tobias, and Marco all come down with Ax's sickness. And now, for the first time, Cassie is on her own...