The Sickness

Ax, an Andalite alien, has fallen ill with yamphut, a virus that leaves him seriously unwell. The Animorphs, the five human teens who can transform into animals, race to find a way to help their ally. The book follows their effort to save Ax while the danger to him grows.

The Sickness is part of K.A. Applegate's Animorphs series, a science fiction adventure for young readers about kids who gain the ability to shapeshift and must defend Earth from hidden alien invaders.