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PokeRus is a status that your Pokémon get. During battle, PokeRus doubles the amount of EVs that you gain from each opponent. If you don't know what EVs are, read the Effort Values section of this guide: https://www.pokecommunity.com/threads/72415.
PokeRus is completely random, there is no telling if you'll get it. But when you do, keep at least one infected Pokémon in your box so that you can get PokeRus whenever you want.
Thank you AB for answering this question. I think it time to close it up.
Next time you have a question, ask it in Simple Questions. Using this thread helps us reduce spam so it helps us help you. The link to this overly helpful thread is in my sig and in the group of stickies near the top of the FR/LG (GBA) thread listing.