http://www.serebii.net/games/pokerus.shtml this should help with if your asking how does pokerus work and happen. But if you mean like how do you get it on your other pokemon put the pokemon you want infected and the infected pokemon in your party. Run in the grass and get into some battles, you can just run or battle the pokemon it doesn't matter. The pokemon you want infected should become infected eventually roughly after about 5 to 10 battles. Hope this helped