Instructions
Look up the GameShark or Action Replay Pokerus cheat code for your game. For example, the code "33969F4A EE0B4CC2" when used with "Pokemon: Sapphire" will make your Pokemon catch Pokerus after each fight. You can find codes for your games at codejunkies.com or GameShark.com.
Obtain a GameShark or Action Replay cheat code device for your console. Make sure you get the correct device for your cheat code. Alternatively, you could play your "Pokemon" game in an emulator such as Visual Boy Advance, No$GBA or Dolphin that supports cheat codes natively. Emulators will allow you to use either Action Replay or GameShark codes interchangeably.
Enter your Pokerus cheat code into your hardware or emulator. Depending on the specific code, you might need to battle wild Pokemon to infect your own. The precise instructions should come with the code.