From the Super Mario Wiki
World 5 is the fifth world in New Super Mario Bros. Wii. The world is a tropical rainforest with poisonous water, which can be seen in some levels. Bramble vines block some of the paths. They will occasionally change positions. There are eight levels in this world, two of them being castle levels that are guarded by Iggy Koopa, and one being a Ghost House.
Various Mushroom Houses are also found around here, giving the player items such as Starmans, Super Mushrooms, and Propeller Mushrooms. On the map, there are Stalking Piranha Plants. Mario and the gang can get attacked by and be forced to play in the Enemy Course. There are a total of three Enemy Courses in the area. This makes it one of the only worlds in the game with several enemy courses, and the one with the most. Also, once Iggy has been defeated in the castle, the bramble patches will disappear completely.
World 5-Ghost House