Instructions
Catch the Pokérus, if one of your Pokémon does not already have it. Catch the virus by battling or capturing a wild Pokémon. There is approximately a one in 21,000 chance that any given wild Pokémon has the Pokérus.
Press the "X" button to pause the game, and then open your Pokémon party list. The infected Pokémon has a status symbol of "PKRS" by its name.
Move the infected Pokémon to the middle-right spot in your party. Pokérus spreads to other Pokémon that are directly next to the infected Pokémon.
Walk around outside in areas where there is tall grass, deep sand or water, and battle the wild Pokémon you find there. After several battles, go to any city and enter the Pokémon Center.
Speak to the nurse at the counter. If one of your other Pokémon has caught the virus, she will alert you.
Continue infecting as many Pokémon as possible. When the Nintendo DS's internal clock reaches midnight, the infected Pokémon will be cured.