Poison (status effect)
Poison is a deadly infection which can affect Mario through various means. Mostly by exotic animals or enemies attacking him. There are some foods that are poisonous too. Poison is mostly used in the RPG games and rarely in other titles.
In Super Mario RPG: Legend of the Seven Stars, this is one of the few conditions the party can inflict on enemies. Bowser is able to do this with his Poison Gas attack. In the Paper Mario series, whenever Mario (or anyone else for that matter) is poisoned they usually turn green. In Mario & Luigi: Superstar Saga, they can turn white, purple, or pink, in order of the severity of the poison. In Mario & Luigi: Partners in Time, poison is indicated by that character's color scheme darkening.
In other games there's an item called Poison Mushroom. If eaten, the Poison effect will be making a character shrink, lose health/become poisoned or die.