Championships do not care about Pokérus. It is useful for training Pokémon, but having Pokérus does not have any effect in online battles.
Pokérus is used to raise Effort Values (EVs) faster than normal. Note that it is not required for EV Training, but just cuts the amount of time required in half. From Bulbapedia :
When a Pokémon has the Pokérus, it gains double the effort values from battling.
However it does not increase the maximum EVs a Pokémon can obtain. This means you can fully EV train your Pokémon without ever having Pokérus. Bulbapedia:
A Fully Trained Pokémon is a Pokémon that has reached 510 EVs overall, the maximum a Pokémon can achieve
It does not matter when a Pokemon gains EVs. Since Generation V, EVs are applied as soon as they are gained, and you do not need to level up in order for them to apply. The formulas for stat totals only care about the EV values, when they changed doesn't matter. Bulbapedia.
Lastly, although Pokerus may appear as a 'status' it does not prevent status conditions
When a Pokémon is infected with Pokérus, their status screen will display an icon indicating this special "status" the same way status conditions would be noted. If the Pokémon becomes affected by a status condition such as Sleep, the Pokérus icon will be temporarily replaced until the status condition is cured.