- Narrator: Cassie
Ax comes down with an Andalite illness that quickly spreads to the rest of the team.
Tropes:
- All Up to You: One by one, nearly all of the Animorphs come down with the Yamphut. First Ax, then Jake, then Rachel, then Marco, and finally Tobias, leaving Cassie to complete the mission on her own.
- Ass Shove: Cassie leaves her dad to take care of a sick Tobias. Apparently, there was a suppository involved.
- Contrived Coincidence: A human-Controller opens the entrance to the Yeerk pool from the outside just before the Gleet BioFilter can kill Cassie, allowing her to escape.
- Damsel in Distress: Aftran 492 has been discovered as a traitor to the Yeerks, and Visser Three plans to personally interrogate her to get the information she has on the Andalite bandits.
- Death Seeker: A Yeerk host tries escaping the Yeerk pool in hopes that the guards will kill him. They just stun him, because killing him would waste a perfectly good host body.
- Dressing as the Enemy: Cassie acquires a Yeerk and infests Mr. Tidwell to get past the Gleet BioFilter and infiltrate the Yeerk pool.
- Dung Fu: Cassie in osprey morph foils a Hunter-Killer robot by dropping bird poop on its lens.
- High-School Dance: The first two chapters focus on one, where Rachel forces Jake and Cassie to go together. A problem arises when Ax goes delirious and starts demorphing and panicking.
- I Want My Mommy!: A five-year-old Yeerk host starts screaming for his mother the moment the Yeerk leaves him to feed.
- I'm Melting!: This is how Cassie describes morphing into a Yeerk, with all of her bones and organs melting away and her remaining flesh turning to mucus.
- Misery Poker: In the last chapter, they engage in some one-upping over who had it worse.Rachel: My mom didn't let me eat any solid food until today. And it's been four days since I got sick.
Tobias: (a hawk) That's the worst thing that happened to you while you were sick? I'm not even sure Cassie's dad is a real vet. He tried to shove a pill up my-
Marco: Yeah, well, my dad brought me baby aspirin from the store. Baby aspirin! Like for a baby. - My Species Doth Protest Too Much: Illim explains that the Yeerk Peace Movement consists of Yeerks who don't want to enslave hosts and would rather have a symbiotic relationship with them.
- No Biochemical Barriers: One can only assume that the disease organisms in Ax's Tria gland are Earth based, as he's been there for one and a half years by then. It would make sense why the ruptured Tria gland would be fatal for him, but only cause the flu in the rest of the Animorphs. What doesn't make sense is why Tobias got sick, since he's a hawk for most of the book.
- Properly Paranoid: Marco knows by now that the Animorphs trying to do something nice and normal always goes wrong. He's proven right when Ax falls ill during the dance.
- Race Against the Clock: Two clocks.
- The first is Visser Three's interrogation of Aftran, which Cassie has to prioritize first.
- The second is the critical moment of the Yamphut, where Ax's Tria gland has to be removed.
- Roadside Surgery: Cassie does have her dad's veterinary clinic on hand to perform surgery on Ax, but it's far from an ideal place for a sick Andalite.
- Screw This, I'm Outta Here: Allison tries hitting on Ax in human morph. A few seconds of Ax's general mannerisms turns her the other way.
- Shapeshifter Mode Lock: Cassie takes a third option to save Aftran by giving her the morphing power, letting her acquire a humpback whale, and letting her go free in the ocean.
- Slippery as an Eel: The Animorphs save Ax acquire freshwater eels to try and get into the Yeerk pool through the water pipes. The eels are too small to be a threat to humans, but they do have a habit of biting each other until the Animorphs get their instincts under control. They're also literally slippery, so Tobias grabbed a talonful from the bait shop and came back with only one because he kept dropping them.
- Sick Episode: All of the Animorphs save for Cassie come down with the Yamphut, or Andalite flu as Marco calls it.
- Too Much Information: Marco goes on a diatribe on how many ways there are to refer to vomiting. Cassie is torn between accepting the levity and being grossed out.
- Ungrateful Bastard: Ax refuses to acknowledge Aftran's role in saving his life. Her role involved infesting him to tell Cassie where the Tria gland is, meaning he woke up with a Yeerk in his head. Aftran is annoyed with him for this, and Cassie has to talk him down once she leaves.
