fight wild Pokemon and like 1/20,000 chances ur Pokemon that u have with u will get it
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It means that that certain Pokemon had pokerus and is no longer able to get pokerus again
Pokerus happens at random, it is about as rare a finding a shiny pokemon.
Pokerus is a virus that infects your Pokemon. It catches this virus from a wild Pokemon that also has pokerus. If you keep a Pokemon with pokerus in you party, your other party members will also get this virus. However, this is a good virus as it doubles the EVs you earn from battles. Your Pokemon will eventually lose pokerus in four days but you can preserve it by keepin it in PC and allowing your other pokemons catching it.
http://www.neoseeker.com/Games/action_replay/
Get a Pokemon with Pokerus and put the Pokemon you want to have it in the party with it. To transfer the PokeRus condition to other Pokemon, have the infected Pokemon in the party with the desired Pokemon and take it out to battle. After a few battles, most (or all) of the Pokemon will have contracted PokeRus.