QWERTY DITTO, hah, no problem.
PokeRus is extremely rare. You have to get obscenely lucky and happen to catch a wild pokemon that has the virus. You infect your team with PokeRus by leaving the pokemon with PokeRus in your team, and eventually the virus spreads throughout your party.
What PokeRus does is it doubles the amount of EVs you get when you KO a Pokémon, so it is very useful in EV training.
Another thing that is important to note: PokeRus only lasts for like 24 or 48 hours of game time when the pokemon is in your party, and once your pokemon no longer has PokeRus, it becomes immune, and cannot catch it again. When the Pokémon is stored in your PC though, it will not lose the disease, so it's smart to infect 2 or 3 extra pokemon, and keep them in your PC at all times, so you always have someone to infect new pokemon you need to EV train.