it is very rare but if you battle a wild pokemon with it (you won't be able to tell) or if you already have one run with it in your party and the pokemon you want infested next to it
Well, very rarely, if you heal ur pokemon, the lady(nurse joy) at the pokemon center counter will say that ur pokemon have the pokerus.
Also, check out the bottem right corner of this page: http://pokemondb.net/ev
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