Pokerus doubles the EV points you gain for defeating a pokemon.
It also adds to the experience gained.
Eg, You defeat a Tyranitar, giving 3 EVs in Attack. If your pokemon had pokerus, you would get 6 EVs in Attack.
When you level up and it says +3 or whatever that is not directly effected by pokerus. If you got a lot more EVs though, it would make a difference, and the increase in stat would be higher.