Instructions
Get a Pokemon that is infected with pokerus. A creature that is cured of pokerus cannot spread the disease to other Pokemon. If you want to get pokerus easily, you can trade a Pokemon for one that is infected with pokerus.
Put the Pokemon that is infected with pokerus in your party. Put the five Pokemon you want to infect with the disease in your party. Walk in the long grass of any route until you encounter a wild Pokemon. After 20 to 50 battles, your whole party will be infected with Pokerus.
Keep at least one Pokemon that is infected with pokerus in your PC. Outside of a PC, a Pokemon will be cured of pokerus within a few days. In a PC, time stops completely. Therefore, a Pokemon will have pokerus forever as long as you keep it in a PC. Use this creature to infect future Pokemon.