The Sickness is the twenty-ninth book in the Animorphs series, written by K.A. Applegate. It is narrated by Cassie.

This article is written from a Real World point of view.


Official Description

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 come down with Ax's sickness. And now, for the first time, Cassie is on her own...

Plot Summary

Cassie and Jake are dragged along by Rachel and the others to the school dance. They have a few good laughs when Ax's unfamiliarity with human culture scares away some of the crowd. The laughter stops, however, when he demorphs involuntarily. It soon becomes apparent that Ax is suffering from an inflamed Tria gland (as well as a fever) - much like the lymphatic tonsils in the human mouth. Unlike humans, though, this gland is located at the back of Ax's brain. If it bursts, it will cause irreversible and fatal damage to the brain. Cassie hides the sick alien in her barn, and Erek the Chee erects a hologram around them both.


Inside cover illustration of Cassie, morphed as a Yeerk, in the Yeerk Pool

At the same time, Mr. Tidwell, one of Cassie's teachers, informs her that Aftran (the Yeerk who made a deal with Cassie previously to never again infest an involuntary host) has been captured by Visser Three. She will be tortured, and will thus be forced to divulge all of her secrets; including that of Cassie and the Animorphs. Cassie endures her friends' thinly veiled blame, but they decide to act. They form a plan to enter the Yeerk pool via the facility's extensive piping system. Tobias brings them eels to morph. The Animorphs morph and enter the pipes, but Jake falls ill from Ax's virus (Although it is established that this virus will just make humans sick rather than having the fatal effects that the Andalite version has on Ax). The plan fails, and they return to the surface - through a fire hose and into a burning building.

That night, Cassie has a bad dream. In the morning, she discovers that Rachel is ill. Mr. Tidwell reveals he shares control of his body with Illim, a Yeerk, and that he is a member of the Yeerk Peace Movement. He tells her the time and location of Aftran's interrogation. Marco is the next to fall ill. Then, while Cassie is with him, Tobias, too, falls ill, flying into one of the barn rafters. To keep Tobias safe, she locks him up next to a golden eagle, much to his dismay.

Cassie is all alone, and she needs to infiltrate the Yeerk pool without setting off the Gleet Bio-Filter, which destroys any creature whose DNA is not entered into the computer controls. Because of this, Illim allows her to acquire his Yeerk DNA and take control of Mr. Tidwell. Cassie and Mr. Tidwell enter the enemy stronghold, and she is disgorged into the Yeerk pool. She finds Aftran's cage, and manages to take control of a voluntary host. She rams the Visser, who drops Aftran. Cassie leaves the girl, dives back into the pool, morphs her osprey, and carries Aftran away, pursued by Visser Three, who has morphed an eyeball with tentacles.

Back at the barn, Cassie learns that Ax is in crisis. She allows Aftran to enter Ax's head and tell her the location of the Tria gland. Cassie makes her incision, and pulls the gland out, saving Ax's life.

A few days later, the other Animorphs have recovered from the illness. Aftran is safe, but in order to keep her alive without access to the Kandrona, the Animorphs allow her to morph a humpback whale - on the condition that she stays in that form as a nothlit.


Featured Characters:

Supporting Characters:



Morpher Morphs acquired Morphs used
Jake Eel Eel, Peregrine Falcon
Rachel Eel Eel, Bald Eagle
Cassie Eel, Yeerk Eel, Yeerk, Osprey, Bottlenose Dolphin
Marco Eel Osprey, Eel
Tobias Eel Human (Main), Eel
Ax -- Human (Main)
Visser Three -- Unnamed Yellow Eyeball-like Creature
Aftran 942 -- Humpback Whale


  • The cover quote is, "Cassie is about to make a change for the better...."
  • The inside cover quote is, "Home, sweet home?"
  • An episode of the Animorphs TV series was based on the part of this book where Cassie morphs into a Yeerk. However, in the TV series, it is Rachel who becomes a Yeerk.
  • This book marks the first time where a Yeerk (Aftran) receives the power to morph in their natural state. Aftran uses the morphing ability to turn into a humpback whale nothlit, promising never to go back as a Yeerk.
  • ANIBASE: New to the series are Mr. Tidwell, a teacher at the Animorphs' school, and the Yeerk in his head, Illim. Mr. Tidwell is a voluntary host to Illim, since Illim is part of the Yeerk Peace Movement. Together they work to help bring down the tyranny of Yeerk slavery from the inside. They are the ones who alert Cassie to Aftran's plight, and it is Illim that Cassie ultimately ends up morphing
  • ANIBASE: The cover of this book was one of the few regular series titles to feature an alien morph. For all the other books, cover artist David B. Mattingly used photographs of the character models and the animals, and then "digitally painted" them in Photoshop. But for these alien morphs, he went back to his pre-computer roots and painted the aliens by hand. The only alien that wasn't completely fabricated was Ax himself -- a model was actually hired to make poses, and David Mattingly painted over him!


International Covers

Taken from

Main Series #1 The Invasion | #2 The Visitor | #3 The Encounter | #4 The Message | #5 The Predator | #6 The Capture | #7 The Stranger | #8 The Alien | #9 The Secret | #10 The Android | #11 The Forgotten | #12 The Reaction | #13 The Change | #14 The Unknown | #15 The Escape | #16 The Warning | #17 The Underground | #18 The Decision | #19 The Departure | #20 The Discovery | #21 The Threat | #22 The Solution | #23 The Pretender | #24 The Suspicion | #25 The Extreme | #26 The Attack | #27 The Exposed | #28 The Experiment | #29 The Sickness | #30 The Reunion | #31 The Conspiracy | #32 The Separation | #33 The Illusion | #34 The Prophecy | #35 The Proposal | #36 The Mutation | #37 The Weakness | #38 The Arrival | #39 The Hidden | #40 The Other | #41 The Familiar | #42 The Journey | #43 The Test | #44 The Unexpected | #45 The Revelation | #46 The Deception | #47 The Resistance | #48 The Return | #49 The Diversion | #50 The Ultimate | #51 The Absolute | #52 The Sacrifice | #53 The Answer | #54 The Beginning
Megamorphs The Andalite's Gift | In the Time of Dinosaurs | Elfangor's Secret | Back to Before
Chronicles The Andalite Chronicles | The Hork-Bajir Chronicles | VISSER | The Ellimist Chronicles
Alternamorphs The First Journey | The Next Passage
TV Tie-Ins Meet the Stars of Animorphs