They gain double EV's, you can stack up the effect with any power items, to gain mass evs (when they cured they have a smiley face) You can have a pokemon with undefined pokerus when you store it in a box, you can infect pokerus by having that pokemon in your party.
Heres some tips:
-Always have at least 1 pokemon with pokerus (to infect the others)
-Train fast the pokemon with pokerus for maximun efficacy