a dude on Yahoo answers gave a Nice explanation, so I'm just going to quote him.
" Indeed, the PokéRus does go away in about 48 hours, however your
Pokémon still gain the boosted EV benefits even after they are cured.
If you wish to keep the virus, put the infected Pokémon in your PC.
This way, the virus will not be cured and you will be able to keep it
When you want to spred it to another Pokémon, simply take your
infected one out, put it in your party and start leveling. The virus
will then pass on to others in your party.
Be sure to put at least one infected Pokémon back in the PC box to
keep the PokéRus!"
pokerus goes away in 48 hours if you dont put it in the PC Box. so make sure to have Your pokerus Pokemon in the PC Box when you dont EV Train it