|This article is written from a Real World point of view.|
- "Look, I want to help you. But three members of the group are sick. Really sick. As in one requiring brain surgery. Isn't there some way the Yeerk peace movement can rescue Aftran without our help?" [...]
"I'm sorry to hear that members of your group are ill. But when the Visser finishes with Aftran, he will know everything. And then every Yeerk in the peace movement will be dead. Their hosts as well. Everyone who has ever helped you will be rounded up and made Controllers. Everyone you care about will be made Controllers. It will all end, Cassie. The defeat will be total, and permanent."
- ―Cassie and Illim
The Sickness, published in April 1999 and written by Melinda Metz based on an outline by K.A. Applegate, is the twenty-ninth book in the Animorphs series. It is the eighth book narrated by Cassie and the fifth book with Cassie as the sole narrator.
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...
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, 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, most likely killing him. Cassie hides the sick alien in her barn, and Erek the Chee erects a hologram around them both.
At the same time, Mr. Tidwell, one of Cassie's teachers, informs her that Aftran 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.
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-Filters, 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 cuts 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 Kandrona Rays, the Animorphs allow her to morph a humpback whale – on the condition that she stays in that form as a nothlit.
- Aftran 942 (Final Appearance)
- Mr. Tidwell (First Appearance) (Only Appearance)
- Illim (First Appearance) (Only Appearance)
- Erek King
- Visser Three
- Hedrick Chapman / Iniss 226
- Various Hork-Bajir, Taxxon and human-Controllers
- David (Mentioned Only)
- Karen (Mentioned Only)
- Estril 731 (Mentioned Only)
- Mr. King (Mentioned Only)
- Peter (Mentioned Only)
- Elfangor-Sirinial-Shamtul (Mentioned Only)
- Forlay-Esgarrouth-Maheen (Mentioned Only)
- Wildlife Rehabilitation Clinic
- Yeerk Pool
- Abandoned Construction Site (Mentioned Only)
- The Gardens (Mentioned Only)
- Hork-Bajir Valley (Mentioned Only)
- Ax contracts yamphut, an Andalite disease threatening to burst a gland at the back of his head and kill him. Ax's illness is also contagious, affecting every other Animorph except Cassie, although they only receive flu-like symptoms.
- Cassie is approached by Mr. Tidwell, one of her teachers, who reveals he is a friend of Aftran 942 and knows who she and her friends are. He informs her that Aftran is be interrogated by Visser Three and subsequently killed.
- The Yeerk Peace Movement is introduced.
- Cassie morphs into a Yeerk to bypass the Gleet Bio-Filters at the Yeerk pool, infesting Tidwell and another host in order to free Aftran.
- Ax is briefly infested by Aftran and becomes a Controller.
- Ax becomes the third Andalite in history to become infested by a Yeerk, following Esplin 9466's partial infestation of Aldrea-Iskillion-Falan and his subsequent control of Alloran-Semitur-Corrass.
- This marks the first instance in the series where a Yeerk receives the power to morph in their natural state, with Aftran using the morphing ability to turn into a humpback whale nothlit.
|Morpher||Morphs acquired||Morphs used|
|Jake||Eel||Peregrine Falcon, Eel|
|Rachel||Eel||Bald Eagle, Eel, Great Horned Owl|
|Tobias||Eel||Human (himself), Eel|
|Cassie||Eel, Yeerk (Illim)||Eel, Osprey, Housefly, Yeerk (Illim), Great Horned Owl (partially), Bottlenose Dolphin (Monica)|
|Marco||Eel||Osprey, Eel, Housefly (possibly)|
|Visser Three||--||Unnamed Yellow Eyeball-like Creature|
|Aftran 942||Humpback Whale||Humpback Whale (becomes trapped in morph)|
- The cover quote is, "Cassie is about to make a change for the better...."
- The inside cover quote is, "Home, sweet home?"
- This is the third time in the series where an Animorph is infested by a Yeerk and becomes a Controller.
- This is the first and only time in the series where an Animorph morphs into a Yeerk and infests someone.
- 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.
- 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!
- The Animorphs fret that Erek King cannot stop Ax if the young Andalite becomes delirious and leaves the Wildlife Rehabilitation Clinic, since it would violate the Chee's prohibition against violence. However, Mr. King was able to successfully restrain Rachel in grizzly bear morph in The Android.
- Cassie receives books about brain surgery from her mother, but then rushes to Mr. Tidwell's house, morphs Illim, goes to the Yeerk pool and frees Aftran, immediately flying to her barn after in order to perform surgery on Ax. As she prepares for the operation, she muses about what she read in the books, even though she never took the time to do so.
- Cassie refers to her regular form as a "human morph" twice, and upon seeing Visser Three, remarks that he is in "Andalite morph."