Pokerus

Infect your team with Pokerus!

I've tested maybe twenty different codes, but here is one that genuinely works.

120776C2 00004288
94000130 FFFE0000
120776C2 00004280
D2000000 00000000

First enter a random wild Pokemon battle (preferably with something weaker that would be easy to capture quickly). Then, when the message comes up saying you've caught the wild Pokemon, press and hold A until the battle ends and you're in normal game play. Release A, then look at your party. If done correctly, some of them should be infected with Pokerus. : D