The Sickness is the twenty-ninth book in the Animorphs

Animorphs

Animorphs is an English language science fiction series of young adult books written by K. A. Applegate and published by Scholastic. Five humans, Jake, Marco, Cassie, Rachel, and Tobias, and one alien, Aximili-Esgarrouth-Isthill , obtain the ability to morph into any animal they touch. They name...

series, authored by K.A. Applegate. It is known to have been ghostwritten

Ghostwriter

A ghostwriter is a professional writer who is paid to write books, articles, stories, reports, or other texts that are officially credited to another person. Celebrities, executives, and political leaders often hire ghostwriters to draft or edit autobiographies, magazine articles, or other written...

 by Melinda Metz

Melinda Metz

Melinda Metz is an American author of young adult books. Her series Roswell High, about teenage aliens, is the basis of The WB television series Roswell. Another series, Fingerprints, is about a psychic girl who reads thoughts from fingerprints...

. It is narrated by Cassie

Cassie (Animorphs)

Cassie is a fictional character from the sci-fi book series Animorphs, written by K. A. Applegate. Her last name is never mentioned, although she referred to it as being "nice" in The Secret.-Biography:...

.

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 (and a fever of 95.5° F) - 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.

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 children morph and enter the pipes, but Jake falls ill from Ax's virus. 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 to it, 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, so 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 little 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

Nothlit

In the fictional Animorphs series by K. A. Applegate, a nothlit is a being trapped in a morph. If one stays in a morph for over two Earth hours, that form becomes permanent. They are unable to use the Escafil device to regain their morphing power...

.

Morphs

Morpher	Morphs Acquired	Morphs Used
Jake Jake Berenson Jake Berenson is a fictional character and one of the main protagonists from the science-fiction book series Animorphs, written by K. A. Applegate. Until the penultimate book of the series, his last name was never revealed, and he was known only as Jake...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
Rachel Rachel (Animorphs) Rachel is a fictional character from the sci-fi book series Animorphs. Her full name is never explicitly mentioned in the books, but her relation to her cousin Jake Berenson through their fathers makes it most likely to be Rachel Berenson, if it was never changed or hyphenated.-Biography:Before the...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
Tobias Tobias (Animorphs) Tobias is a fictional character from the sci-fi book series Animorphs written by K.A. Applegate. His surname is never mentioned, but had the Ellimist not altered time in order to take his father, Elfangor-Sirinial-Shamtul, back to his old life, his full name would have been Tobias Fangor, the...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
Marco Marco (Animorphs) Marco is a fictional character from the sci-fi book series, Animorphs, written by K. A. Applegate. His last name is never mentioned.-Biography:...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...	Osprey Osprey The Osprey , sometimes known as the sea hawk or fish eagle, is a diurnal, fish-eating bird of prey. It is a large raptor, reaching more than in length and across the wings... , Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
Cassie Cassie (Animorphs) Cassie is a fictional character from the sci-fi book series Animorphs, written by K. A. Applegate. Her last name is never mentioned, although she referred to it as being "nice" in The Secret.-Biography:...	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators... , Yeerk Yeerk Yeerks are a fictional extraterrestrial species from the book and television series "Animorphs" written by K. A. Applegate and published by Scholastic.-Biology:...  (Illim)	Eel Eel Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators... , Yeerk Yeerk Yeerks are a fictional extraterrestrial species from the book and television series "Animorphs" written by K. A. Applegate and published by Scholastic.-Biology:... , Osprey
Ax Aximili-Esgarrouth-Isthill Aximili-Esgarrouth-Isthill is a fictional character from the sci-fi book series Animorphs.-Biography:Aximili is a young Andalite who becomes stranded on Earth when the Andalite Dome Ship GalaxyTree is shot down by Yeerk fighters...		Human Human Humans are the only living species in the Homo genus...
Visser Three Visser Three Visser Three, born as Esplin 9466 Primary and later known as Visser One, is a fictional character and the main antagonist of the sci-fi book series Animorphs, written by K.A. Applegate. He is the leader of the Yeerk forces on Earth, having inherited the planet from Edriss 562, who was Visser One at...		'Eyeball Creature'
Aftran	Humpback Whale Humpback Whale The humpback whale is a species of baleen whale. One of the larger rorqual species, adults range in length from and weigh approximately . The humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the...	Humpback Whale Humpback Whale The humpback whale is a species of baleen whale. One of the larger rorqual species, adults range in length from and weigh approximately . The humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the...

TV series

An episode was base on the part when Cassie morphs into a Yeerk. Although in the tv series, it is Rachel who morphs a yeerk.