From the Super Mario Wiki
An Invincishroom is a mushroom made in Little Fungitown in Mario & Luigi: Superstar Saga. It is given out as a prize for doing well on Star 'Stache Smash. According to the proprietor of the arcade, it is made from Starmen and 1-Up Mushrooms from the Mushroom Kingdom, though these claims are unsubstantiated. When Mario ate one, he contracted Bean Fever, a disease which affects people who are not accustomed to the food of the Beanbean Kingdom. This disease causes the afflicted to become a Bean in three days. Luigi had to travel to Guffawha Ruins alone to find the cure, which is Crabbie Grass.