When selecting the Pokemon you want, at the bottom of the list, there should be an option that says "What Pokemon?". You write the name of the Pokemon you want. This way you can look for Pokemon you haven't seen yet (like, say, Xerneas for Y).
Though, like I said, since nobody can transfer yet, you won't find any.
As for Pokerus, I don't know how it works exactly, all I've heard is that it spreads to any Pokemon in the same box as the infected one. Might have to check.