It can be caused by several moves most of which are Poison-type. Poison and Steel-type Pokémon are immune to being poisoned, making it the only non-volatile status ailment to be ineffective against more than one type.
A poisoned Pokémon will lose 1/8 of its maximum HP every turn in battle.
Along with the poison status, a Pokémon can also be badly poisoned; the effects are similar to poison but instead of having a set amount of damage dealt each turn, badly poisoned Pokémon will receive increasing amounts of damage each turn.
A badly poisoned Pokémon will lose 1/16 of its maximum HP on the first turn, after which damage will increase by 1/16 at the end of every turn