The Android — K.A. Applegate

When Marco runs into his old friend Erek, he doesn't think too much of it. He's got a couple of more important things to do. Like helping to save the world. But then Marco finds out Erek's been hanging with the kids at The Sharing. And he starts to think that something just a little weird is going on.

So Marco, Jake, and Ax decide to morph and check old Erek out. Just to see if he's been infested with a Yeerk. The good news is that Erek's not a human-Controller. The bad news is that Erek's not even human. . . .