The most reliable way I found is to enter a battle with the PKRS pokemon at the front of your party, then switch to the non-PKRS pokemon in battle. Nearly every time I tried it, the other pokemon got the PKRS straight after the battle.
Just put a pokemon with pokerus in your party! As you walk, the virus will spread it self! I think...
The method I used was to battle a wild Pokemon with a POKErus infected Pokemon and whichever Pokemon is second on your team will get the virus after the battle
