Fever Swamp is a deadly swamp located somewhere in Florida.
The Werewolf of Fever Swamp
In the book The Werewolf of Fever Swamp, Will Blake mentions to Grady Tucker that the swamp gets its name from the rumored fever it harbors. The fever will make the victim get high temperatures before going insane and running into the swamp, to never leave again. The swamp is otherwise home to the isolated Swamp Hermit, a bog that acts like quicksand, and the titular Werewolf.
Goosebumps HorrorLand (Video Game)
In the video game Goosebumps HorrorLand, Fever Swamp is the name of an area in HorrorLand. Unlike the titular swamp from The Werewolf of Fever Swamp, this version of the swamp is one of HorrorLand's water parks. The park hosts magic shows that are run by Horrifico and feature The Great Gargantua.