Instructions
Battle a Pokemon with Pokerus in order to infect your Pokemon with the virus. Encountering a Pokemon with Pokerus is completely random, but once you have defeated one, the nurse at any Pokemon Center will inform you that your Pokemon is infected.
Place your Pokemon in the PC if you wish to temporarily freeze the 24 hours you have with the virus.
Withdraw any Pokemon you wish to infect from the PC. Leave an open slot for the infected Pokemon.
Withdraw the infected Pokemon. The other five Pokemon in your party will now be infected. Repeat with as many Pokemon as you wish until the 24 hours have elapsed.