Body Mushroom

Body Mushroom (カラダカラキノコガハエルダケ Karada kara kinoko ga haeru dake, lit. "Mushrooms Growing from the Body Mushroom", Parasite Mushroom in the FUNimation subs ) is a species of mushroom that grows on Amazon Lily. If a person eats the mushroom, many will start growing on their body which can be fatal. To cure that, one has to pull the mushrooms out of the person, followed by the area being burned to destroy the roots. Luffy ate one thinking it was a Wairidrake, but he was soon unconscious and covered in mushrooms so the Kujas pulled them out and burned him.

Trivia

 * The medical treatment of "burning" the mushroom to get it off is indeed a natural cure for several biological dieases that grow on the skin. However this is normally done with acids rather then fire as the Kuja used; though in some cultures burning the skin with fire was indeed used as a cure for some diseases.