The name of the Animorph book series, by K. A. Applegate. Five
teenagers are granted
powers to 'morph' or become other animals by a dying alien Andalite prince named
Elfangor, killed by his enemy, Visser Three, an intelligent parasite called
a Yeerk inhabiting another Andalite's body.
The teenagers, Jake, Rachel, Tobias, Cassie, and Marco, use their new powers to fight the spreading
Yeerk infection on earth. They later teem up with Elfangor's brother, Aximili.
The book series encompasses 54 books, 4 Megamorphs, 4 other books detailing the history of different people/races
mentioned, and several other Alternamorphs, which
are choose-your-own-ending books.