[Naix]

yoohoo, take Flu the Gulpin. For its entire lifetime, Flu has been suffering from a chronic sickness that causes it to vomit (read: use a Poison-type move) once every 5 turns it's in battle, starting with the first one. Note: The counter is reset every time Flu enters a battle. Also, items speed up the cycle to the point where Flu has to use a Poison-type move once every 3 turns, starting with the next one, until the end of the battle. Note: Items used outside of battle do not have this effect if Flu is leading your party. Because of this, its fellow Gulpin (as well as Poison-types in general) avoided it like the plague, on the possibility that Flu's illness might be contagious. However, Flu thought they were scared of it because they were weaker than it (which they may or may not actually be), and because of this, if Flu enters a battle against any other Poison-type, it must solo it. Note: This doesn't mean that you have to switch it in against other Poison-types.

Have fun!