Wow, I didn't think you could get pokerus in the battle frontier! Anyway, I got pokerus a long time ago. Really make EV training much easier with macho brace. Actually, when I was trying to find a modest ditto, I captured two, and the both had pokerus. That's the third ditto I caught with it. I'm starting to think that isn't coincidental.
My Blaziken, Matang, Groudon, and Manectric caught pokerus around Rustboro City on Sapphire a long time ago. At the time I didn't know what on earth it was and turned off the game without saving. I really wish I kept it now and I haven't seen pokerus since that incident. Also, my game is not altered by any type of cheating device.
The PokéRus virus will double the number of EVs that a Pokémon will earn. When a Pokémon with the PokéRus is given a Macho Brace to hold, it will earn 4x more EVs. When a Pokémon gets the PokéRus, it will have a period of time where it can spread the virus to other Pokémon. You will know if a Pokémon can spread the virus if you check it's summary screen, as there is a little box saying "PKRS" in the lower-left corner, where status effects, like Paralysis "PAR" and Poison "PSN", etc are displayed. Once a Pokémon catches the PokéRus, the effects of it will last forever. However, after a day, the Pokémon will no longer be able to spread the virus, you will know when this has happened, because when you check the Pokémon's status screen, the "PKRS" will be replaced with a small black dot. To spread the PokéRus to other Pokémon, simply put the Pokémon with PokéRus in your party, and fight a couple of battles with trainers or wild Pokémon, and gradually, the other Pokémon should catch it. It seems to work best if you put the PokéRus Pokémon at the top. If you get the PokéRus, make sure to deposit at least one Pokémon that has the active "spreadable" PokéRus in one of the PC boxes. Pokémon that have the PokéRus will not lose the ability to spread PokéRus if they are kept in a PC box.