"Last, but definitely not the least, isn’t an item, but rather a special virus
called the Pokerus. Wild Pokemon have a very, VERY small chance of possessing
this very special virus (almost as rare as wild Pokemon being shiny), but if
you’re lucky enough to battle one, the Pokemon that beat it will be infected by
the virus. To find out, simply open up your Pokemon’s page, and if a purple bar
with “PKRS” appears next to its name, it’s infected. After a few more battles,
your entire party will be infected with this really awesome virus.
What does it do? It doubles the amount of EVs you receive in battle, taking
into consideration all the items mentioned above.
So if you battle a Starly with a Macho Brace AND Pokerus, instead of getting
one Speed EV like you normally should, you get (1 x 2) x 2 = 4 EVs.
And if you battle that same Starly with a Power Anklet AND Pokerus, instead of
getting one Speed EV like you normally should, you get (1 + 4) x 2 = 10 EVs.
One of the users of this guide has given me proof that this is the correct
formula, so yeah. =)
They don’t work with the vitamins though. Feeding a Pokemon with Protein
doesn’t give it 20 Attack EVs, but the usual 10.
And one more thing, Pokerus is never-ending, but the spreading part doesn’t
last for more than one or two days. Everytime the date rolls over (a new day),
Pokerus “leaves” your Pokemon. When it does, the purple “PKRS” sign will be
replaced by an orange smiley face. Don’t fret, though, as the double in effort
points will still be in effect - the only thing that leaves is the ability to
spread it to other Pokemon.
With this in mind, one must avoid losing Pokerus completely by storing a
Pokemon with an active Pokerus on the PC Storage. This will preserve the
status of Pokerus forever until the Pokemon is taken out. A handy thing,
indeed."
http://www.gamefaqs.com/portable/ds/file/946308/56146