All you have to do is battle a wild pokemon that has it(ofc there's no way to tell until you're infected). I think you have to actually battle and defeat the wild pokemon to get the pokerus, but I'm not sure. Catching it will also work.
Trying to get Pokerus intentionally is probably a waste of time unless you get EXTREMELY lucky. Pretty much everyone who gets Pokerus is just lucky enough to get it by accident, like I was.
"R.I.P Grammer, you shall be missed"-penguin_god_777 Official Boney of the Mother 3 board
I'm fairly certain you can contract it just from battling a wild pokemon that's infected with it. I've had some really good luck with PokeRus and almost all of my Pokemon in White 2 have it, so I really hope GF succeeds in making a way to send them to XY.
Playing) Fire Emblem: Awakening, Guild Wars 2 Waiting) Pokemon XY