Instructions
Play Pokemon as usual. If you fight a wild Pokemon with Pokerus, you won't know at the time. However, after healing your party at a Pokemon Center, the game will tell you that one of your Pokemon has caught the virus.
Open the menu and view your current Pokemon. Go to each one, selecting it and choosing to look at its info.
Look in the lower-left corner of the info window to see any current status effects. A Pokemon carrying Pokerus will display a purple icon reading "PKRS."
Carry a Pokemon with Pokerus in your party to allow it to spread to the other Pokemon in your group. Check the info screen regularly to see which members have caught it.
Check back on Pokemon already carrying Pokerus. After a few days the virus will wear off. Instead of "PKRS" you will see a small happy face by the Pokemon's picture. This indicates the Pokemon is now immune to Pokerus and will not spread the virus, but it will continue to gain the its positive effect indefinitely.